New shutdown.h file

The new shutdown.h file includes the shutdown modes and the functions
to shutdown Window Maker.

The function definitios were moved from funcs.h and the struct WShutdownMode
from WindowMaker.h.

Now, externs are not needed, only include shutdown.h

src/shutdown.c

@@ -37,6 +37,7 @@
 #include "winspector.h"
 #include "wmspec.h"
 #include "colormap.h"
+#include "shutdown.h"
 
 extern Atom _XA_WM_DELETE_WINDOW;
 extern Time LastTimestamp;