smbd: pass a dirfsp to fd_open() and rename it to fd_openat()

smbd: pass a dirfsp to fd_open() and rename it to fd_openat()

For now no change in behaviour as all callers still pass conn->cwd_fsp. This
just prepared fd_openat() to deal with real dirfsp's pass by callers later on
when adding calls to fd_openat(dirfspm ...) in the directory enumeration loop.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>