Add publish-subscribe sockets to AF_UNIX (filesystem sockets).

The idea of a publish-subscribe socket (as opposed to a traditional socket) is that anything written to one end is read out by all processes reading from the other end.

This raises some issues that need attention; most notably, if a process doesn't read data sent to it, how long do we wait (or how much data do we allow to accumulate) before dropping the data or disconnecting that process?

It seems that one would want to be able to have both SOCK_STREAM and SOCK_DGRAM publish/subscribe channels, so it isn't entirely clear yet how best to graft this functionality into the socket API. Or the socket code. It might end up needing to be its own thing instead, or needing to extend the socket API, which would be unfortunate but perhaps unavoidable.

The goal for these sockets is to provide a principled and robust scheme for passing notices around. This will allow replacing multiple existing ad hoc callback schemes (such as for notices of device insertions) and also to allow various signaling schemes among user processes that have never been very workable in Unix. Those who remember AREXX will recognize the potential; but of course it will take time before anything like that can be orchestrated.

For this reason it's important that it be reasonably easy for one of the endpoints to be inside the kernel; but it's also important that peer credentials and permissions work.

Another goal is to be able to use publish/subscribe sockets to provide a compatibility library for Linux desktop software that wants to use dbus or any successor to dbus.

I'm marking this project hard as it's still only about half baked.