Now that the high level networking API is coming together, we should also add unix domain helpers. Should be pretty straightforward.

Some notes here: #73 (comment)

I guess something like:

async def open_unix_stream(path):
    ...

async def open_unix_listeners(path, *, mode=0o666, backlog=None):
    ...

mode=0o666 matches what twisted does; tornado does 0o600. Should research which is better as a default.

The biggest issue is to figure out what to do about unlink-on-close. It's nice to keep thing tidy, but it introduces a race condition if you're trying to do no-downtime upgrades...