Move headers from server/include to include/maxscale

- Headers now to be included as <maxscale/xyz.h>
- First step, no cleanup of headers has been made. Only moving
  from one place to another + necessary modifications.

include/maxscale/externcmd.h

@@ -0,0 +1,41 @@
+#ifndef _EXTERN_CMD_HG
+#define _EXTERN_CMD_HG
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl.
+ *
+ * Change Date: 2019-07-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include <unistd.h>
+#include <string.h>
+#include <errno.h>
+#include <maxscale/skygw_utils.h>
+#include <maxscale/log_manager.h>
+#include <maxscale/maxscale_pcre2.h>
+
+#define MAXSCALE_EXTCMD_ARG_MAX 256
+
+typedef struct extern_cmd_t
+{
+    char** argv; /*< Argument vector for the command, first being the actual command
+                * being executed. */
+    int n_exec; /*< Number of times executed */
+    pid_t child; /*< PID of the child process */
+} EXTERNCMD;
+
+char* externcmd_extract_command(const char* argstr);
+EXTERNCMD* externcmd_allocate(char* argstr);
+void externcmd_free(EXTERNCMD* cmd);
+int externcmd_execute(EXTERNCMD* cmd);
+bool externcmd_substitute_arg(EXTERNCMD* cmd, const char* re, const char* replace);
+bool externcmd_can_execute(const char* argstr);
+bool externcmd_matches(const EXTERNCMD* cmd, const char* match);
+
+#endif