Cleanup header files

- All now include maxscale/cdefs.h as the very first file.
- MXS_[BEGIN|END]_DECLS added to all C-headers.
  Strictly speaking not necessary for private headers, but
  does not hurt either.
- Include guards moved to the very top of the file.
- #pragma once added.

include/maxscale/limits.h

@@ -1,3 +1,4 @@
+#pragma once
 #ifndef _MAXSCALE_LIMITS_H
 #define _MAXSCALE_LIMITS_H
 /*
@@ -13,6 +14,10 @@
  * Public License.
  */
 
+#include <maxscale/cdefs.h>
+
+MXS_BEGIN_DECLS
+
 // This file defines hard limits of MaxScale.
 
 // Thread information is stored in a bitmask whose size must be a
@@ -21,4 +26,6 @@
 // multiple of 8 minus 1.
 #define MXS_MAX_THREADS 255
 
+MXS_END_DECLS
+
 #endif