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/config_runtime.c

@@ -17,6 +17,8 @@
 #include <maxscale/gwdirs.h>
 #include <maxscale/spinlock.h>
 
+#include "maxscale/service.h"
+
 static SPINLOCK crt_lock = SPINLOCK_INIT;
 
 bool runtime_link_server(SERVER *server, const char *target)