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/gw_authenticator.h

@@ -1,5 +1,6 @@
-#ifndef GW_AUTHENTICATOR_H
-#define GW_AUTHENTICATOR_H
+#pragma once
+#ifndef _MAXSCALE_GW_AUTHENTICATOR_H
+#define _MAXSCALE_GW_AUTHENTICATOR_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -27,12 +28,15 @@
  * @endverbatim
  */
 
+#include <maxscale/cdefs.h>
 #include <maxscale/buffer.h>
 #include <openssl/crypto.h>
 #include <openssl/ssl.h>
 #include <openssl/err.h>
 #include <openssl/dh.h>
 
+MXS_BEGIN_DECLS
+
 /** Maximum number of authenticator options */
 #define AUTHENTICATOR_MAX_OPTIONS 256
 
@@ -134,5 +138,7 @@ typedef enum
 bool authenticator_init(void **instance, const char *authenticator, const char *options);
 char* get_default_authenticator(const char *protocol);
 
+MXS_END_DECLS
+
 #endif /* GW_AUTHENTICATOR_H */