Fix for win32 stat() problems

Attached is a patch that attempts to fix the issues with stat() not
properly updating st_size on win32, as reported in this thread:
http://archives.postgresql.org/pgsql-hackers/2008-03/msg01181.php

It has to have a chance to affect things beyond just the
pg_relation_size() function, so this patch fixes all cases where we do
stat() and use the st_size member. It doesn't change all occurances of
stat() since I didn't want to incur the double filesystem lookups
unnecessary cases.

Any objections?

//Magnus