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/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