Store persisted config directory

The persisted configuration subdirectory will be used to store changes to
the configuration. The gwdirs.h header now supports setting and getting
the value for this directory.

server/core/config.c

@@ -65,6 +65,7 @@
 #include <maxscale/service.h>
 #include <maxscale/spinlock.h>
 #include <maxscale/utils.h>
+#include <maxscale/gwdirs.h>
 
 typedef struct duplicate_context
 {