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/server.cc

@@ -41,10 +41,10 @@
 #include <maxscale/http.hh>
 #include <maxscale/maxscale.h>
 
-#include "maxscale/monitor.h"
-#include "maxscale/poll.h"
-#include "maxscale/workertask.hh"
-#include "maxscale/worker.hh"
+#include "internal/monitor.h"
+#include "internal/poll.h"
+#include "internal/workertask.hh"
+#include "internal/worker.hh"
 
 using maxscale::Semaphore;
 using maxscale::Worker;