Moving header functions to main.h

The functions of main.c should be included in main.h, not in funcs.h.
This patch adds the main.h file and moves the function prototypes to
this file.

The not needed "include funcs.h" are removed.

src/shutdown.c

@@ -31,6 +31,7 @@
 #include "window.h"
 #include "client.h"
 #include "funcs.h"
+#include "main.h"
 #include "properties.h"
 #include "session.h"
 #include "winspector.h"