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/test/test_modulecmd.cc

@@ -22,7 +22,7 @@
 #include <maxscale/session.h>
 #include <maxscale/json_api.h>
 
-#include "../maxscale/monitor.h"
+#include "../internal/monitor.h"
 
 #define TEST(a, b) do{if (!(a)){printf("%s:%d " b "\n", __FILE__, __LINE__);return 1;}}while(false)