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

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.)