pgsql: Generalize TRUNCATE to support truncating multiple tables in one

Log Message:
-----------
Generalize TRUNCATE to support truncating multiple tables in one
command.  This is useful because we can allow truncation of tables
referenced by foreign keys, so long as the referencing table is
truncated in the same command.

Alvaro Herrera

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        truncate.sgml (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/truncate.sgml.diff?r1=1.17&r2=1.18)
    pgsql/src/backend/catalog:
        heap.c (r1.279 -> r1.280)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/heap.c.diff?r1=1.279&r2=1.280)
    pgsql/src/backend/commands:
        tablecmds.c (r1.143 -> r1.144)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.143&r2=1.144)
    pgsql/src/backend/nodes:
        copyfuncs.c (r1.295 -> r1.296)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c.diff?r1=1.295&r2=1.296)
        equalfuncs.c (r1.234 -> r1.235)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c.diff?r1=1.234&r2=1.235)
    pgsql/src/backend/parser:
        gram.y (r2.481 -> r2.482)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.481&r2=2.482)
    pgsql/src/backend/tcop:
        utility.c (r1.232 -> r1.233)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c.diff?r1=1.232&r2=1.233)
    pgsql/src/include/catalog:
        heap.h (r1.72 -> r1.73)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/heap.h.diff?r1=1.72&r2=1.73)
    pgsql/src/include/commands:
        tablecmds.h (r1.21 -> r1.22)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/tablecmds.h.diff?r1=1.21&r2=1.22)
    pgsql/src/include/nodes:
        parsenodes.h (r1.271 -> r1.272)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h.diff?r1=1.271&r2=1.272)
    pgsql/src/test/regress/expected:
        temp.out (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/temp.out.diff?r1=1.9&r2=1.10)
        truncate.out (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/truncate.out.diff?r1=1.9&r2=1.10)
    pgsql/src/test/regress/sql:
        temp.sql (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/temp.sql.diff?r1=1.5&r2=1.6)
        truncate.sql (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/truncate.sql.diff?r1=1.2&r2=1.3)