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/log_manager.cc

@@ -36,7 +36,7 @@
 #include <maxscale/session.h>
 #include <maxscale/spinlock.h>
 #include <maxscale/utils.h>
-#include "maxscale/mlist.h"
+#include "internal/mlist.h"
 
 #define MAX_PREFIXLEN 250
 #define MAX_SUFFIXLEN 250