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/skygw_utils.cc

@@ -27,7 +27,7 @@
 #include <regex.h>
 #include <maxscale/debug.h>
 #include <sys/time.h>
-#include "maxscale/skygw_utils.h"
+#include "internal/skygw_utils.h"
 #include <maxscale/atomic.h>
 #include <maxscale/random_jkiss.h>
 #include <pcre2.h>