desktop-shell: Make zapping configurable

Add a new boolean weston.ini option, "allow-zap" to enable or disable
the Ctrl-Alt-Backspace key combination.

Signed-off-by: Bob Ham <bob.ham@collabora.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
diff --git a/man/weston.ini.man b/man/weston.ini.man
index 4f97c4d..6ebe68a 100644
--- a/man/weston.ini.man
+++ b/man/weston.ini.man
@@ -258,6 +258,10 @@
 .B none.
 By default, no animation is used.
 .TP 7
+.BI "allow-zap=" true
+whether the shell should quit when the Ctrl-Alt-Backspace key combination is
+pressed
+.TP 7
 .BI "binding-modifier=" ctrl
 sets the modifier key used for common bindings (string), such as moving
 surfaces, resizing, rotating, switching, closing and setting the transparency