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/externcmd.h

@@ -1,5 +1,6 @@
-#ifndef _EXTERN_CMD_HG
-#define _EXTERN_CMD_HG
+#pragma once
+#ifndef _MAXSCALE_EXTERN_CMD_HG
+#define _MAXSCALE_EXTERN_CMD_HG
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -13,6 +14,7 @@
  * Public License.
  */
 
+#include <maxscale/cdefs.h>
 #include <unistd.h>
 #include <string.h>
 #include <errno.h>
@@ -20,6 +22,8 @@
 #include <maxscale/log_manager.h>
 #include <maxscale/pcre2.h>
 
+MXS_BEGIN_DECLS
+
 #define MAXSCALE_EXTCMD_ARG_MAX 256
 
 typedef struct extern_cmd_t
@@ -38,4 +42,6 @@ bool externcmd_substitute_arg(EXTERNCMD* cmd, const char* re, const char* replac
 bool externcmd_can_execute(const char* argstr);
 bool externcmd_matches(const EXTERNCMD* cmd, const char* match);
 
+MXS_END_DECLS
+
 #endif