More cleanup of monitor.h

Move some more functions into core header + miscellaneous cleanup

server/core/modulecmd.c

@@ -12,12 +12,15 @@
  */
 
 #include <maxscale/modulecmd.h>
+
 #include <maxscale/alloc.h>
 #include <maxscale/config.h>
 #include <maxscale/pcre2.h>
 #include <maxscale/platform.h>
 #include <maxscale/spinlock.h>
+
 #include "maxscale/filter.h"
+#include "maxscale/monitor.h"
 
 /** Size of the error buffer */
 #define MODULECMD_ERRBUF_SIZE 512