[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/j6.module

@@ -11,6 +11,7 @@ j6 = module("j6",
         "memutils.cpp",
         "mutex.cpp",
         "protocol_ids.cpp",
+        "protocols/service_locator.cpp",
         "syscalls.s.cog",
         "sysconf.cpp.cog",
         "syslog.cpp",
@@ -27,6 +28,7 @@ j6 = module("j6",
         "j6/memutils.h",
         "j6/protocols.h",
         "j6/protocols/service_locator.h",
+        "j6/protocols/service_locator.hh",
         "j6/syscalls.h.cog",
         "j6/sysconf.h.cog",
         "j6/syslog.hh",