pgsql: Implement function-local GUC parameter settings, as per recent

Log Message:
-----------
Implement function-local GUC parameter settings, as per recent discussion.
There are still some loose ends: I didn't do anything about the SET FROM
CURRENT idea yet, and it's not real clear whether we are happy with the
interaction of SET LOCAL with function-local settings.  The documentation
is a bit spartan, too.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        catalogs.sgml (r2.155 -> r2.156)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml?r1=2.155&r2=2.156)
    pgsql/doc/src/sgml/ref:
        alter_function.sgml (r1.13 -> r1.14)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_function.sgml?r1=1.13&r2=1.14)
        create_function.sgml (r1.75 -> r1.76)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_function.sgml?r1=1.75&r2=1.76)
    pgsql/src/backend/access/transam:
        xact.c (r1.246 -> r1.247)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c?r1=1.246&r2=1.247)
    pgsql/src/backend/catalog:
        pg_aggregate.c (r1.86 -> r1.87)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/pg_aggregate.c?r1=1.86&r2=1.87)
        pg_proc.c (r1.145 -> r1.146)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/pg_proc.c?r1=1.145&r2=1.146)
    pgsql/src/backend/commands:
        functioncmds.c (r1.83 -> r1.84)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/functioncmds.c?r1=1.83&r2=1.84)
        proclang.c (r1.72 -> r1.73)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/proclang.c?r1=1.72&r2=1.73)
    pgsql/src/backend/optimizer/util:
        clauses.c (r1.247 -> r1.248)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c?r1=1.247&r2=1.248)
    pgsql/src/backend/parser:
        gram.y (r2.600 -> r2.601)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.600&r2=2.601)
    pgsql/src/backend/utils:
        Gen_fmgrtab.sh (r1.36 -> r1.37)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/Gen_fmgrtab.sh?r1=1.36&r2=1.37)
    pgsql/src/backend/utils/fmgr:
        fmgr.c (r1.108 -> r1.109)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/fmgr/fmgr.c?r1=1.108&r2=1.109)
    pgsql/src/backend/utils/init:
        miscinit.c (r1.162 -> r1.163)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/miscinit.c?r1=1.162&r2=1.163)
        postinit.c (r1.176 -> r1.177)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/postinit.c?r1=1.176&r2=1.177)
    pgsql/src/backend/utils/misc:
        guc.c (r1.414 -> r1.415)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.414&r2=1.415)
    pgsql/src/bin/pg_dump:
        pg_dump.c (r1.471 -> r1.472)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c?r1=1.471&r2=1.472)
    pgsql/src/include/catalog:
        catversion.h (r1.419 -> r1.420)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.419&r2=1.420)
        pg_attribute.h (r1.131 -> r1.132)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_attribute.h?r1=1.131&r2=1.132)
        pg_class.h (r1.101 -> r1.102)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_class.h?r1=1.101&r2=1.102)
        pg_proc.h (r1.465 -> r1.466)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.465&r2=1.466)
    pgsql/src/include/utils:
        guc.h (r1.83 -> r1.84)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/guc.h?r1=1.83&r2=1.84)
    pgsql/src/test/regress/expected:
        guc.out (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/guc.out?r1=1.4&r2=1.5)
    pgsql/src/test/regress/sql:
        guc.sql (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/guc.sql?r1=1.4&r2=1.5)