pgsql: Support INSERT/UPDATE/DELETE RETURNING in plpgsql, with rowcount

Log Message:
-----------
Support INSERT/UPDATE/DELETE RETURNING in plpgsql, with rowcount checking
as per yesterday's proposal.  Also make things a tad more orthogonal by
adding the recent STRICT addition to EXECUTE INTO.
Jonah Harris and Tom Lane

Modified Files:
--------------
    pgsql/doc/src/sgml:
        plpgsql.sgml (r1.98 -> r1.99)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.98&r2=1.99)
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.94 -> r1.95)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.94&r2=1.95)
        pl_exec.c (r1.174 -> r1.175)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.174&r2=1.175)
        pl_funcs.c (r1.53 -> r1.54)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_funcs.c.diff?r1=1.53&r2=1.54)
        plpgsql.h (r1.78 -> r1.79)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.78&r2=1.79)
        scan.l (r1.53 -> r1.54)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/scan.l.diff?r1=1.53&r2=1.54)
    pgsql/src/test/regress/expected:
        plpgsql.out (r1.50 -> r1.51)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out.diff?r1=1.50&r2=1.51)
    pgsql/src/test/regress/sql:
        plpgsql.sql (r1.43 -> r1.44)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql.diff?r1=1.43&r2=1.44)