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/modules/routing/binlogrouter/test/testbinlog.c

@@ -49,7 +49,7 @@
 #include <maxscale/version.h>
 
 // This isn't really a clean way of testing
-#include "../../../../core/maxscale/service.h"
+#include "../../../../core/internal/service.h"
 
 static void printVersion(const char *progname);
 static void printUsage(const char *progname);