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().

misc.c

@@ -333,6 +333,8 @@ const char *nullseat_get_x_display(Seat *seat) { return NULL; }
 bool nullseat_get_windowid(Seat *seat, long *id_out) { return false; }
 bool nullseat_get_window_pixel_size(
     Seat *seat, int *width, int *height) { return false; }
+StripCtrlChars *nullseat_stripctrl_new(
+    Seat *seat, BinarySink *bs, bool cr, wchar_t sub) { return NULL; }
 
 void sk_free_peer_info(SocketPeerInfo *pi)
 {