Make plpgsql's unreserved keywords more unreserved.

There were assorted places where unreserved keywords were not treated the
same as T_WORD (that is, a random unrecognized identifier).  Fix them.
It might not always be possible to allow this, but it is in all these
places, so I don't see any downside.

Per gripe from Jim Wilson.  Arguably this is a bug fix, but given the lack
of other complaints and the ease of working around it (just quote the
word), I won't risk back-patching.