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/shmlog.h

@@ -8,8 +8,8 @@
  * default (ringbuffer for storing the debug log).
  *
  */
-#ifndef _I3_SHMLOG_H
-#define _I3_SHMLOG_H
+#ifndef I3_I3_SHMLOG_H
+#define I3_I3_SHMLOG_H
 
 #include <stdint.h>
 #include <pthread.h>