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().

windows/wincons.c

@@ -388,6 +388,12 @@ 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);
+}
+
 static void console_data_untrusted(HANDLE hout, const char *data, size_t len)
 {
     DWORD dummy;

windows/window.c

@@ -320,6 +320,12 @@ bool win_seat_get_window_pixel_size(Seat *seat, int *x, int *y)
     return true;
 }
 
+StripCtrlChars *win_seat_stripctrl_new(Seat *seat, BinarySink *bs_out,
+                                       bool permit_cr, wchar_t substitution)
+{
+    return stripctrl_new_term(bs_out, permit_cr, substitution, term);
+}
+
 static size_t win_seat_output(
     Seat *seat, bool is_stderr, const void *, size_t);
 static bool win_seat_eof(Seat *seat);
@@ -347,6 +353,7 @@ static const SeatVtable win_seat_vt = {
     nullseat_get_x_display,
     nullseat_get_windowid,
     win_seat_get_window_pixel_size,
+    win_seat_stripctrl_new,
 };
 static Seat win_seat_impl = { &win_seat_vt };
 Seat *const win_seat = &win_seat_impl;

windows/winplink.c

@@ -105,6 +105,7 @@ 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 }};