nttrans.c: Fix change_notify. Queue processing code wasn't changing to

nttrans.c: Fix change_notify. Queue processing code wasn't changing to
           the correct directory before doing the stat. Doh !
uid.c: Fix for performance in security=share mode. Invalid vuid meant
       that in security=share mode Samba was always doing the become_user/
       undebome_user pairs for *every* smb. This code fixes it, but tridge
       should review for security implications.
Jeremy.
(This used to be commit c3663379fdcec487feea2e5d848ee012ee6c6baf)