win32 locale fixes

The following patch fixes locale support under win32.

* Saves and reloads LC_COLLATE and LC_CTYPE when a new backend is
execed. Also preserved in pgstat even though it's supposedly not used
there at the moment, to be on the safe side for the future. With this
patch, passes regression tests with regional settings set to != english.

* This is not necessary (see previous discussion) to do for the other
LC_xxx types. These are all set in GUC, and are handled by the routines
to save/restore all nondefault GUC varilables.

* setlocale(LC_MESSAGES,"") returns NULL on win32. And it's always
called as part of GUC setup. Patch changes code to ignore the result if
we set it to "". (Can someone confirm that this is not totally unsafe?
From what I can tell, LC_MESSAGS is only set to ""

Oh, and how do I change a locale:d system into english? Mine picks up
swedish from the environment...

After this, the only remaining regression error on win32 is the join one
(the "unimportant ordering" one)

//Magnus