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/readconnroute/readconnection.h

@@ -1,3 +1,4 @@
+#pragma once
 #ifndef _READCONNECTION_H
 #define _READCONNECTION_H
 /*
@@ -25,8 +26,12 @@
  *
  * @endverbatim
  */
+
+#include <maxscale/cdefs.h>
 #include <maxscale/dcb.h>
 
+MXS_BEGIN_DECLS
+
 /**
  * Internal structure used to define the set of backend servers we are routing
  * connections to. This provides the storage for routing module specific data
@@ -84,4 +89,7 @@ typedef struct router_instance
     struct router_instance
         *next;
 } ROUTER_INSTANCE;
+
+MXS_END_DECLS
+
 #endif