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.

server/core/maxkeys.c

@@ -24,10 +24,10 @@
  */
 #include <getopt.h>
 #include <stdio.h>
-#include <secrets.h>
-#include <skygw_utils.h>
-#include <log_manager.h>
-#include <gwdirs.h>
+#include <maxscale/secrets.h>
+#include <maxscale/skygw_utils.h>
+#include <maxscale/log_manager.h>
+#include <maxscale/gwdirs.h>
 
 struct option options[] =
 {