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/maxadmin.h

@@ -1,5 +1,6 @@
-#ifndef _MAXADMIN_H
-#define _MAXADMIN_H
+#pragma once
+#ifndef _MAXSCALE_MAXADMIN_H
+#define _MAXSCALE_MAXADMIN_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -13,6 +14,10 @@
  * Public License.
  */
 
+#include <maxscale/cdefs.h>
+
+MXS_BEGIN_DECLS
+
 #define MAXADMIN_DEFAULT_SOCKET                "/tmp/maxadmin.sock"
 
 #define MAXADMIN_CONFIG_DEFAULT_SOCKET_TAG_LEN 7
@@ -28,4 +33,6 @@
 #define MAXADMIN_AUTH_PASSWORD_PROMPT          "PASSWORD"
 #define MAXADMIN_AUTH_PASSWORD_PROMPT_LEN      8
 
+MXS_END_DECLS
+
 #endif