Split the service header into internal and external parts

The service header in include/maxscale/ contains the public part of the
service API. These functions can be safely used by the modules.

The internal header located in service/core/maxscale/ is used by the core
to initialize MaxScale at startup or to provide other services in a more
controlled way (the config_runtime, for example).

server/core/service.c

@@ -48,7 +48,6 @@
 #include <math.h>
 #include <fcntl.h>
 #include <maxscale/session.h>
-#include <maxscale/service.h>
 #include <maxscale/gw_protocol.h>
 #include <maxscale/listener.h>
 #include <maxscale/server.h>
@@ -68,6 +67,8 @@
 #include <maxscale/alloc.h>
 #include <maxscale/utils.h>
 
+#include "maxscale/service.h"
+
 /** Base value for server weights */
 #define SERVICE_BASE_SERVER_WEIGHT 1000