pgsql: Mark to_char(timestamp without timezone) as stable, not

Log Message:
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Mark to_char(timestamp without timezone) as stable, not immutable, since its
result now depends on the lc_messages setting, as noted by Bruce.
Also, mark to_number() and the numeric-type variants of to_char() as stable,
because their results depend on lc_numeric; this is a longstanding oversight.
Also, mark to_date() and to_char(interval) as stable; although these appear
not to depend on any GUC variables as of CVS HEAD, that seems a property
unlikely to survive future improvements. It seems best to mark all the
formatting functions stable and be done with it.
catversion not bumped, because this does not seem critical enough to force
a post-RC1 initdb, and anyway we cannot do so in the back branches.

Modified Files:
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pgsql/src/include/catalog:
pg_proc.h (r1.428 -> r1.429)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.428&r2=1.429)