Add support for connected INET sockets

[Dan Smith]

This is an update of the previous set to apply on top of v19-rc1 and to
address some of the trivial changes suggested on list.  Those are detailed
per-patch.  I will follow up with a patch to change ckpt_debug to ckpt_err
where appropriate, across the whole net/*/checkpoint.c set.  Since this
will apply to the unchanged gso patch as well, I decided to do this as a
follow-on cleanup patch.