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/httprequest.cc

@@ -11,8 +11,8 @@
  * Public License.
  */
 
-#include "maxscale/httprequest.hh"
-#include "maxscale/admin.hh"
+#include "internal/httprequest.hh"
+#include "internal/admin.hh"
 
 #include <ctype.h>
 #include <string.h>