SOCK_SEQPACKET support

[vi]

There should be a mode of UnixStream/UnixListener on Linux where it uses SOCK_SEQPACKET instead of SOCK_STREAM.

[vi]

It's also on the way into std.

[rotty]

Unfortunatly, the integration into std was declined due to MacOS not supporting SOCK_SEQPACKET. I'm considering assembling a PR to add SOCK_SEQPACKET support to mio-uds (along the lines of the std PR), however I'd like to know if such a PR would be welcome in mio-uds, or if it would be declined for the same reason -- @alexcrichton?

[vi]

Just make it Linux-specific API.

It's not a question whether Rust (including async) should support SOCK_SEQPACKET or not; it should. The question is how: in this crate, in std, in separate crate, gated by Cargo feature...