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.

server/modules/routing/readwritesplit/readwritesplit.h

@@ -1,3 +1,4 @@
+#pragma once
 #ifndef _RWSPLITROUTER_H
 #define _RWSPLITROUTER_H
 /*
@@ -24,10 +25,13 @@
  * @endverbatim
  */
 
+#include <maxscale/cdefs.h>
 #include <maxscale/dcb.h>
 #include <maxscale/hashtable.h>
 #include <math.h>
 
+MXS_BEGIN_DECLS
+
 #undef PREP_STMT_CACHING
 
 #if defined(PREP_STMT_CACHING)
@@ -356,5 +360,6 @@ typedef struct router_instance
 #define BACKEND_TYPE(b) (SERVER_IS_MASTER((b)->backend_server) ? BE_MASTER :    \
         (SERVER_IS_SLAVE((b)->backend_server) ? BE_SLAVE :  BE_UNDEFINED));
 
+MXS_END_DECLS
 
 #endif /*< _RWSPLITROUTER_H */