pgsql: Make plpgsql's unreserved keywords more unreserved.

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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/bac95fd4740e3ac13baf5c2ad38b9c9dc26f9c9d

Modified Files
--------------
src/pl/plpgsql/src/gram.y | 57 ++++++++++++++++++++++++++++++++++++--
src/pl/plpgsql/src/pl_scanner.c | 19 +++++++++++++
src/pl/plpgsql/src/plpgsql.h | 1 +
3 files changed, 74 insertions(+), 3 deletions(-)