Code added to fix the renaming of a directory under NT SMB calls.

Code added to fix the renaming of a directory under NT SMB calls.
local.h: Changed MAXDIR to MAX_OPEN_DIRECTORIES - shmem size also tuned by this.
dir.c: Use MAX_OPEN_DIRECTORIES.
nttrans.c: Allow opening of a directory to succeed. Doesn't actually open
           a file descriptor but takes a files_struct slot marked as an
           fd.
reply.c: Changed to close any outstanding is_directory files.
         reply_close changed to understand directory files.
server.c: Added open_directory(), close_directory() calls.
smb.h: Added is_directory to files_struct.
       Changed OPEN_FNUM to check that target is !is_directory (this
       prevents the normal file calls from processing a directory
       files_struct.
Jeremy.
(This used to be commit e01ce693f47e75e277f3440d46e32b0bd866b550)