Fix plpgsql's EXIT so that an EXIT without a label only matches a loop,

Fix plpgsql's EXIT so that an EXIT without a label only matches a loop,
never a BEGIN block.  This is required for Oracle compatibility and is
also plainly stated to be the behavior by our original documentation
(up until 8.1, in which the docs were adjusted to match the code's behavior;
but actually the old docs said the correct thing and the code was wrong).

Not back-patched because this introduces an incompatibility that could
break working applications.  Requires release note.