pgsql: Fix up foreign-key mechanism so that there is a sound semantic

Log Message:
-----------
Fix up foreign-key mechanism so that there is a sound semantic basis for the
equality checks it applies, instead of a random dependence on whatever
operators might be named "=".  The equality operators will now be selected
from the opfamily of the unique index that the FK constraint depends on to
enforce uniqueness of the referenced columns; therefore they are certain to be
consistent with that index's notion of equality.  Among other things this
should fix the problem noted awhile back that pg_dump may fail for foreign-key
constraints on user-defined types when the required operators aren't in the
search path.  This also means that the former warning condition about "foreign
key constraint will require costly sequential scans" is gone: if the
comparison condition isn't indexable then we'll reject the constraint
entirely. All per past discussions.

Along the way, make the RI triggers look into pg_constraint for their
information, instead of using pg_trigger.tgargs; and get rid of the always
error-prone fixed-size string buffers in ri_triggers.c in favor of building up
the RI queries in StringInfo buffers.

initdb forced due to columns added to pg_constraint and pg_trigger.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        catalogs.sgml (r2.144 -> r2.145)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml.diff?r1=2.144&r2=2.145)
        trigger.sgml (r1.49 -> r1.50)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/trigger.sgml.diff?r1=1.49&r2=1.50)
    pgsql/src/backend/catalog:
        aclchk.c (r1.136 -> r1.137)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/aclchk.c.diff?r1=1.136&r2=1.137)
        dependency.c (r1.63 -> r1.64)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/dependency.c.diff?r1=1.63&r2=1.64)
        heap.c (r1.316 -> r1.317)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/heap.c.diff?r1=1.316&r2=1.317)
        index.c (r1.278 -> r1.279)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c.diff?r1=1.278&r2=1.279)
        namespace.c (r1.91 -> r1.92)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/namespace.c.diff?r1=1.91&r2=1.92)
        pg_constraint.c (r1.34 -> r1.35)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/pg_constraint.c.diff?r1=1.34&r2=1.35)
        pg_conversion.c (r1.34 -> r1.35)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/pg_conversion.c.diff?r1=1.34&r2=1.35)
    pgsql/src/backend/commands:
        conversioncmds.c (r1.30 -> r1.31)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/conversioncmds.c.diff?r1=1.30&r2=1.31)
        explain.c (r1.153 -> r1.154)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c.diff?r1=1.153&r2=1.154)
        tablecmds.c (r1.213 -> r1.214)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.213&r2=1.214)
        trigger.c (r1.212 -> r1.213)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c.diff?r1=1.212&r2=1.213)
        typecmds.c (r1.99 -> r1.100)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/typecmds.c.diff?r1=1.99&r2=1.100)
    pgsql/src/backend/tcop:
        utility.c (r1.271 -> r1.272)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c.diff?r1=1.271&r2=1.272)
    pgsql/src/backend/utils/adt:
        ri_triggers.c (r1.90 -> r1.91)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ri_triggers.c.diff?r1=1.90&r2=1.91)
        ruleutils.c (r1.248 -> r1.249)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c.diff?r1=1.248&r2=1.249)
    pgsql/src/backend/utils/cache:
        lsyscache.c (r1.147 -> r1.148)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/lsyscache.c.diff?r1=1.147&r2=1.148)
        syscache.c (r1.110 -> r1.111)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/syscache.c.diff?r1=1.110&r2=1.111)
    pgsql/src/bin/pg_dump:
        pg_dump.c (r1.459 -> r1.460)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c.diff?r1=1.459&r2=1.460)
    pgsql/src/bin/psql:
        describe.c (r1.150 -> r1.151)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/describe.c.diff?r1=1.150&r2=1.151)
    pgsql/src/include/catalog:
        catversion.h (r1.383 -> r1.384)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.383&r2=1.384)
        indexing.h (r1.97 -> r1.98)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/indexing.h.diff?r1=1.97&r2=1.98)
        pg_constraint.h (r1.24 -> r1.25)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_constraint.h.diff?r1=1.24&r2=1.25)
        pg_trigger.h (r1.26 -> r1.27)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_trigger.h.diff?r1=1.26&r2=1.27)
    pgsql/src/include/commands:
        trigger.h (r1.60 -> r1.61)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/trigger.h.diff?r1=1.60&r2=1.61)
    pgsql/src/include/utils:
        lsyscache.h (r1.116 -> r1.117)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/lsyscache.h.diff?r1=1.116&r2=1.117)
        rel.h (r1.96 -> r1.97)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/rel.h.diff?r1=1.96&r2=1.97)
        syscache.h (r1.67 -> r1.68)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/syscache.h.diff?r1=1.67&r2=1.68)
    pgsql/src/test/regress/expected:
        alter_table.out (r1.100 -> r1.101)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/alter_table.out.diff?r1=1.100&r2=1.101)
        foreign_key.out (r1.41 -> r1.42)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/foreign_key.out.diff?r1=1.41&r2=1.42)
    pgsql/src/test/regress/sql:
        alter_table.sql (r1.55 -> r1.56)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/alter_table.sql.diff?r1=1.55&r2=1.56)
        foreign_key.sql (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/foreign_key.sql.diff?r1=1.17&r2=1.18)