cage: return exit code of primary client

Some applications indicate different shutdown conditions by returning
specific exit codes. One of these is e.g. Kodi, which returns 64 in case
the user chose "Power off" and 66 in case the user chose "Reboot".
In order to act on these exit codes, it thus makes sense in some
situations to pass them on from the primary client to the caller of
Cage.

This exit code is only returned if the primary client is the cause of
Cage terminating.

Co-authored-by: Patrick Steinhardt <ps@pks.im>

server.h

@@ -53,6 +53,7 @@ struct cg_server {
 
 	bool xdg_decoration;
 	bool allow_vt_switch;
+	bool return_app_code;
 };
 
 #endif