Add a new seat method to return the cursor position.

The motivation is for the SUPDUP protocol.  The server may send a
signal for the terminal to reset any input buffers.  After this, the
server will not know the state of the terminal, so it is required to
send its cursor position back.

psftp.c

@@ -65,6 +65,7 @@ static const SeatVtable psftp_seat_vt = {
     nullseat_set_trust_status_vacuously,
     cmdline_seat_verbose,
     nullseat_interactive_yes,
+    nullseat_get_cursor_position,
 };
 static Seat psftp_seat[1] = {{ &psftp_seat_vt }};