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().

unix/gtkwin.c

@@ -351,6 +351,13 @@ static bool gtk_seat_get_window_pixel_size(Seat *seat, int *w, int *h)
     return true;
 }
 
+StripCtrlChars *gtk_seat_stripctrl_new(Seat *seat, BinarySink *bs_out,
+                                       bool permit_cr, wchar_t substitution)
+{
+    GtkFrontend *inst = container_of(seat, GtkFrontend, seat);
+    return stripctrl_new_term(bs_out, permit_cr, substitution, inst->term);
+}
+
 static void gtk_seat_notify_remote_exit(Seat *seat);
 static void gtk_seat_update_specials_menu(Seat *seat);
 static void gtk_seat_set_busy_status(Seat *seat, BusyStatus status);
@@ -380,6 +387,7 @@ static const SeatVtable gtk_seat_vt = {
     gtk_seat_get_windowid,
 #endif
     gtk_seat_get_window_pixel_size,
+    gtk_seat_stripctrl_new,
 };
 
 static void gtk_eventlog(LogPolicy *lp, const char *string)

unix/uxcons.c

@@ -465,6 +465,12 @@ static void console_eventlog(LogPolicy *lp, const char *string)
         console_logging_error(lp, string);
 }
 
+StripCtrlChars *console_stripctrl_new(Seat *seat, BinarySink *bs_out,
+                                      bool permit_cr, wchar_t substitution)
+{
+    return stripctrl_new(bs_out, permit_cr, substitution);
+}
+
 /*
  * Special functions to read and print to the console for password
  * prompts and the like. Uses /dev/tty or stdin/stderr, in that order

unix/uxplink.c

@@ -403,6 +403,7 @@ static const SeatVtable plink_seat_vt = {
     nullseat_get_x_display,
     nullseat_get_windowid,
     nullseat_get_window_pixel_size,
+    console_stripctrl_new,
 };
 static Seat plink_seat[1] = {{ &plink_seat_vt }};