Support for unix sockets

[cmkarlsson]

It would be great if glisten can support unix sockets and abstract unix sockets.

Erlang supports this by passing the option {ip, {local, Path}} to gen_tcp:listen where Path is either a file system path, or an abstract unix sockets where the Path starts with null byte (<<0,path:utf8>>).

I have implemented a prototype in glisten where this is implemented. The prototype makes a couple of different modifications to achieve this:

1. Extending IpAddress to also represent a UnixSocketAddress
2. modifying bind to see if the incoming interface starts with "/" or "@" (which is the convention to deal with unix sockets, however we can change this to use something other than bind, like: fn bind_unix_socket(handler, path: String) -> Handler instead
3. Modifying the type of sockname from #(Dynamic, Int) to Dynamic as the underlying erlang code might return {Hostname :: binary(), Port :: int()} | {local, Path :: binary()}. Then this dynamic is decoded into either #(ipv4|ipv6, port) or #(path, 0).

From my point of view the biggest problem with the prototype is that it piggy-backs on some naming conventions like IpAddress and the return type from sockname which might not be ideal.

Would it be OK if I do the PR with what I currently have and we can take discussion from there?

[rawhat]

Hey, thanks for your interest in this! I have a local branch attempting to address this exact issue, but wasn't super thrilled with the API I came up with.

If you want to open a PR, I'd be more than happy to look at it!

[rawhat]

For reference, this was the original ask: rawhat/mist#48