uxpty: send seat_eof when the pty master gives EIO.

The uxpty backend is going to be reused to implement the "session"
channel type in the upcoming SSH server implementation, which puts
quite a few new requirements on it. The first of them is that when we
get EOF from the subprocess's output channel (or rather, EIO from the
pty), we should actually notify the Seat of this.

In principle we should have been doing this all along, I'm pretty
sure. It hasn't happened to matter until now because the receiving
Seats haven't done much with that notification. But it will matter
when that's what controls the sending of SSH_MSG_CHANNEL_EOF.

unix/uxpty.c

@@ -678,6 +678,7 @@ void pty_real_select_result(Pty *pty, int event, int status)
 	    seat_stdout(pty->seat, message, strlen(message));
 	}
 
+        seat_eof(pty->seat);
 	seat_notify_remote_exit(pty->seat);
     }
 }