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/authenticator.cc

@@ -15,7 +15,7 @@
 #include <maxscale/modutil.h>
 #include <maxscale/alloc.h>
 
-#include "maxscale/modules.h"
+#include "internal/modules.h"
 /**
  * @file authenticator.c - Authenticator module functions
  */