Restructuring of the code to remove dos_ChDir/dos_GetWd and re-vector them

Restructuring of the code to remove dos_ChDir/dos_GetWd and re-vector them
through the VFS. All file access/directory access code in smbd should now
go via the vfs. Added vfs_chown/vfs_chmod calls. Still looking at vfs_get_nt_acl()
vfs_set_nt_acl() call API design.
Jeremy.
(This used to be commit f96625ec124adb6e110dc54632e006b3620a962b)