pgsql: Fix plpgsql to not treat INSERT INTO as an INTO-variables clause

Log Message:
-----------
Fix plpgsql to not treat INSERT INTO as an INTO-variables clause anywhere
in the string, not just at the start. Per bug #4629 from Martin Blazek.

Back-patch to 8.2; prior versions don't have the problem, at least not in
the reported case, because they don't try to recognize INTO in non-SELECT
statements. (IOW, this is really fallout from the RETURNING patch.)

Tags:
----
REL8_3_STABLE

Modified Files:
--------------
pgsql/src/pl/plpgsql/src:
gram.y (r1.108 -> r1.108.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.108&r2=1.108.2.1)