Implement i3 --get-socketpath, useful for IPC scripts

In order to not depend on X11 just for getting the socket paths, scripts or
other programs can now use i3 --get-socketpath. Since i3 must be present on the
computer anyways, this saves one dependency :).

include/all.h

@@ -65,5 +65,6 @@
 #include "ewmh.h"
 #include "assignments.h"
 #include "regex.h"
+#include "libi3.h"
 
 #endif