net::UnixListener: Consider exposing poll_accept()

[akshayknarayan]

UnixListener::poll_accept() is currently pub(crate).

However, TcpListener::poll_accept() is pub as of this commit: 8a7e577#diff-a0e934bf38b64e9a75c741bff132d8adL129

The corresponding Incoming types do expose poll_accept(), but the UnixListener and TcpListener should perhaps be made consistent - i.e., either restore pub(crate) to TcpListener::poll_accept() or make UnixListener::poll_accept() pub.

[kleimkuhler]

@akshayknarayan I believe this is "temporary" until Tokio no longer depends on mio-uds for UnixListener since it does not have a stable release. If so, I think this could made pub once Tokio depends on a newer version of mio.

[de-vri-es]

This has been fixed by #2845 and #2880 :D

Not released yet, but this issue can probably be closed now.