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/protocol/HTTPD/httpd.h

@@ -1,3 +1,6 @@
+#pragma once
+#ifndef _HTTPD_H
+#define _HTTPD_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -18,6 +21,7 @@
  * 08-07-2013   Massimiliano Pinto  Added HTTPD protocol header file
  */
 
+#include <maxscale/cdefs.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -35,6 +39,8 @@
 #include <maxscale/atomic.h>
 #include <maxscale/gw.h>
 
+MXS_BEGIN_DECLS
+
 #define HTTPD_SMALL_BUFFER 1024
 #define HTTPD_METHOD_MAXLEN 128
 #define HTTPD_USER_MAXLEN 128
@@ -59,3 +65,7 @@ typedef struct httpd_session
     char *query_string;             /*< the Query string, starts with ?, after path_info and document name */
     int headers_received;               /*< All the headers has been received, if 1 */
 } HTTPD_session;
+
+MXS_END_DECLS
+
+#endif