socket.bind() is a blocking call

[njsmith]

...for UNIX domain sockets, since it has to do filesystem traversal.

And for non-UNIX-domain sockets, it also is if you pass in the host or port by name. So far we avoided this by making all the methods on trio.socket.socket only accept "pre-resolved" host/port combos. Which is a bit awkward; really the main motivation was that bind was logically a non-blocking call, so it can't call getaddrinfo, and then I made this the rule across-the-board to keep things consistent. But maybe it would be better to instead make bind async, and then drop the whole "pre-resolution" thing and if the user passes us an unresolved address, just silently resolve it for them.

[njsmith]

I guess there are two options for handling this:

Option 1: Always use run_in_worker_thread for bind. Let Python do whatever resolution it wants to do. This is nice because it lets us dump our pre-resolution code (at least for local addresses). Problems: this way you can't cancel bind, because bind has side-effects. This is awkward if its off in a long-running getaddrinfo; really for AF_INET{,6} sockets we want cancellation to take effect if it's still in the getaddrinfo part, which is slow and has no side-effects, but not if it's in the final bind part part, which is fast and has side-effects. Also, it would mean our regular getaddrinfo replacement machinery (#159) wouldn't work.

Option 2: keep our "pre-resolving" code, but move it internal to bind, and likewise for the other functions that take addresses. Run the actual bind call in a worker thread iff self.family == AF_UNIX (and the given address is not a Linux abstract address). This is a bit risky (we're trying to replicate Python's internal code for this kind of thing), but probably works fine in practice. I guess we might have to do this.

[njsmith]

Since currently, writing explicit bind calls is the only way to make a network server in trio, this actually is a somewhat disruptive compatibility break. Maybe we should get the high-level listener APIs into 0.2.0 so people can start migrating to that before making this change.