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/monitor/mysqlmon/mysql_mon.cc

@@ -32,7 +32,7 @@
 #include <maxscale/mysql_utils.h>
 #include <maxscale/utils.h>
 // TODO: For monitorAddParameters
-#include "../../../core/maxscale/monitor.h"
+#include "../../../core/internal/monitor.h"
 
 /** Column positions for SHOW SLAVE STATUS */
 #define MYSQL55_STATUS_MASTER_LOG_POS 5