pgsql: Add a function lastval(), which returns the value returned by the

Log Message:
-----------
Add a function lastval(), which returns the value returned by the
last nextval() or setval() performed by the current session. Update the
docs, add regression tests, and bump the catalog version. Patch from
Dennis Bj�rklund, various improvements by Neil Conway.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        func.sgml (r1.251 -> r1.252)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.251&r2=1.252)
    pgsql/src/backend/commands:
        sequence.c (r1.122 -> r1.123)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/sequence.c.diff?r1=1.122&r2=1.123)
    pgsql/src/include/catalog:
        catversion.h (r1.272 -> r1.273)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.272&r2=1.273)
        pg_proc.h (r1.363 -> r1.364)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.363&r2=1.364)
    pgsql/src/include/commands:
        sequence.h (r1.31 -> r1.32)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/sequence.h.diff?r1=1.31&r2=1.32)
    pgsql/src/test/regress/expected:
        sequence.out (r1.6 -> r1.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/sequence.out.diff?r1=1.6&r2=1.7)
    pgsql/src/test/regress/sql:
        sequence.sql (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/sequence.sql.diff?r1=1.2&r2=1.3)