[srv.init] Serve a service locator protocol from init

The init process now serves as a service locator for its children,
passing all children a mailbox handle on which it is serving the service
locator protocol.

src/user/srv.init/init.module

@@ -8,6 +8,7 @@ init = module("srv.init",
         "loader.cpp",
         "main.cpp",
         "modules.cpp",
+        "service_locator.cpp",
         "start.s",
     ])