don’t use reversed identifiers for include guards (Thanks Markus)

Done with:

    sed -in 's/\(ifndef\|define\) _\([0-9A-Z_]*\)$/\1 I3_\2/' include/**/*.h

fixes #804

include/ipc.h

@@ -7,8 +7,8 @@
  * ipc.c: UNIX domain socket IPC (initialization, client handling, protocol).
  *
  */
-#ifndef _IPC_H
-#define _IPC_H
+#ifndef I3_IPC_H
+#define I3_IPC_H
 
 #include <ev.h>
 #include <stdbool.h>