[kernel] Make mailbox non-fixed-length again

Going back to letting mailboxes use variable-length data. Note that this
requires extra copies, so shared memory channels should be used for
anything in the hot path. But this allows better RPC over mailboxes and
other flexibility.

Other changes:
- added a j6::proto::sl::client class to act as a service locator
  client, instead of duplicating that code in every program.
- moved protocol ids into j6/tables/protocols.inc so that C++ clients
  can easily have their own API

src/libraries/j6/protocols/service_locator.cpp

@@ -0,0 +1,64 @@
+#include <j6/errors.h>
+#include <j6/protocols/service_locator.hh>
+#include <j6/syscalls.h>
+#include <j6/syslog.hh>
+
+#ifndef __j6kernel
+
+namespace j6::proto::sl {
+
+client::client(j6_handle_t slp_mb) :
+    m_service {slp_mb}
+{
+}
+
+j6_status_t
+client::register_service(uint64_t proto_id, j6_handle_t handle)
+{
+    uint64_t tag = j6_proto_sl_register;
+    size_t handle_count = 1;
+    size_t data = proto_id;
+    size_t data_size = sizeof(proto_id);
+
+    j6_status_t s = j6_mailbox_call(m_service, &tag,
+        &data, &data_size, data_size,
+        &handle, &handle_count);
+
+    if (s != j6_status_ok)
+        return s;
+
+    if (tag == j6_proto_base_status)
+        return data; // contains a status
+
+    return j6_err_unexpected;
+}
+
+j6_status_t
+client::lookup_service(uint64_t proto_id, j6_handle_t &handle)
+{
+    uint64_t tag = j6_proto_sl_find;
+    size_t handle_count = 1;
+    size_t data = proto_id;
+    size_t data_size = sizeof(proto_id);
+    handle = j6_handle_invalid;
+
+    j6::syslog("Looking up service for %x", proto_id);
+    j6_status_t s = j6_mailbox_call(m_service, &tag,
+        &data, &data_size, data_size,
+        &handle, &handle_count);
+
+    if (s != j6_status_ok)
+        return s;
+
+    if (tag == j6_proto_sl_result)
+        return j6_status_ok; // handle is already in `handle`
+
+    else if (tag == j6_proto_base_status)
+        return data; // contains a status
+
+    return j6_err_unexpected;
+}
+
+
+} // namespace j6::proto::sl
+#endif // __j6kernel