Minor cleanup of config.[h|c]

- Add const where applicable.
- Cleanup header files.

Fix collateral damage (statistics.c) as well.

server/core/statistics.c

@@ -24,9 +24,10 @@
  */
 
 #include <maxscale/statistics.h>
+#include <string.h>
 #include <maxscale/alloc.h>
 #include <maxscale/config.h>
-#include <string.h>
+#include <maxscale/debug.h>
 #include <maxscale/platform.h>
 
 static int thread_count = 0;