Move headers from server/include to include/maxscale

- Headers now to be included as <maxscale/xyz.h>
- First step, no cleanup of headers has been made. Only moving
  from one place to another + necessary modifications.

server/core/externcmd.c

@@ -11,7 +11,7 @@
  * Public License.
  */
 
-#include <externcmd.h>
+#include <maxscale/externcmd.h>
 #include <maxscale/alloc.h>
 
 /**