Add a Seat vtable method to get a stripctrl.

If centralised code like the SSH implementation wants to sanitise
escape sequences out of a piece of server-provided text, it will need
to do it by making a locale-based StripCtrlChars if it's running in a
console context, or a Terminal-based one if it's in a GUI terminal-
window application.

All the other changes of behaviour needed between those two contexts
are handled by providing reconfigurable methods in the Seat vtable;
this one is no different. So now there's a new method in the Seat
vtable that will construct a StripCtrlChars appropriate to that kind
of seat. Terminal-window seats (gtkwin.c, window.c) implement it by
calling the new stripctrl_new_term(), and console ones use the locale-
based stripctrl_new().

psftp.c

@@ -61,6 +61,7 @@ static const SeatVtable psftp_seat_vt = {
     nullseat_get_x_display,
     nullseat_get_windowid,
     nullseat_get_window_pixel_size,
+    console_stripctrl_new,
 };
 static Seat psftp_seat[1] = {{ &psftp_seat_vt }};