Fix in-source builds

The internal header directory conflicted with in-source builds causing a
build failure. This is fixed by renaming the internal header directory to
something other than maxscale.

The renaming pointed out a few problems in a couple of source files that
appeared to include internal headers when the headers were in fact public
headers.

Fixed maxctrl in-source builds by making the copying of the sources
optional.

server/core/statistics.cc

@@ -23,7 +23,7 @@
  * @endverbatim
  */
 
-#include "maxscale/statistics.h"
+#include "internal/statistics.h"
 #include <string.h>
 #include <maxscale/alloc.h>
 #include <maxscale/config.h>