compositor-{drm, fbdev, rpi}: Make VT switching configurable

Add a new boolean weston.ini option, "vt-switching" to enable or
disable Ctrl-Alt-Fn key combinations.

Signed-off-by: Bob Ham <bob.ham@collabora.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>

(Derek Foreman changed the prototype for switch_vt_binding to
have a weston_keyboard * instead of weston_seat *.  The pointer
wasn't used, so this is just a warning fix.)
diff --git a/src/compositor.h b/src/compositor.h
index 2848674..130b258 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -751,6 +751,8 @@
 	int32_t kb_repeat_rate;
 	int32_t kb_repeat_delay;
 
+	bool vt_switching;
+
 	clockid_t presentation_clock;
 	int32_t repaint_msec;