Just like in mesg(1) in biff(1) we just need to push down pledge(2) a little

Just like in mesg(1) in biff(1) we just need to push down pledge(2) a little
bit to get the tty name. After this we can unveil(2) the tty with rw access in
order to stat(2)/chmod(2) it, once this is done we can put back the same
pledge(2) just right afterwards.

OK deraadt@