[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/libraries/j6/j6.module

@@ -2,8 +2,10 @@
 
 j6 = module("j6",
     kind = "lib",
+    deps = [ "util" ],
     sources = [
         "init.cpp",
+        "protocol_ids.cpp",
         "syscalls.s.cog",
         "sysconf.cpp.cog",
     ],
@@ -12,6 +14,8 @@ j6 = module("j6",
         "j6/errors.h",
         "j6/flags.h",
         "j6/init.h",
+        "j6/protocols.h",
+        "j6/protocols/service_locator.h",
         "j6/syscalls.h.cog",
         "j6/sysconf.h.cog",
         "j6/types.h",

src/libraries/j6/j6/protocols.h

@@ -0,0 +1,12 @@
+#pragma once
+/// \file j6/protocols.h
+/// Common definition for j6 user-mode IPC protocols
+
+enum j6_proto_base_tag
+{
+    j6_proto_base_status,
+    j6_proto_base_get_proto_id,
+    j6_proto_base_proto_id,
+
+    j6_proto_base_first_proto_id /// The first protocol-specific ID
+};

src/libraries/j6/j6/protocols/service_locator.h

@@ -0,0 +1,14 @@
+#pragma once
+/// \file j6/protocols/service_locator.h
+/// Definitions for the service locator protocol
+
+#include <j6/protocols.h>
+
+extern const uint64_t j6_proto_sl_id;
+
+enum j6_proto_sl_tag
+{
+    j6_proto_sl_register = j6_proto_base_first_proto_id,
+    j6_proto_sl_find,
+    j6_proto_sl_result,
+};

src/libraries/j6/protocol_ids.cpp

@@ -0,0 +1,4 @@
+#include <util/hash.h>
+
+extern "C"
+const uint64_t j6_proto_sl_id = "jsix.protocol.service_locator"_id;