Raise listen backlog for Unix sockets to match std

[Mark-Simulacrum]

Mio uses 1024 rather than -1 to listen by default in a few cases:

• mio/src/net/tcp/listener.rs

  Line 81 in 9a9d691

  listen(&listener.inner, 1024)?;

• mio/src/sys/unix/uds/listener.rs

  Line 19 in 9a9d691

  syscall!(listen(fd, 1024))?;

std has moved to using -1 or equivalent to prefer the largest possible backlog instead, and mio should probably match that? It would help ensure that applications moving between creating sockets in std and in mio get mostly equivalent behavior for this queue.

[Thomasdezeeuw]

Linux doesn't even consider the parameter anymore. I'm not sure about other OS. But I agree we should match std lib, so let's change it.

[Mark-Simulacrum]

Why do you say that Linux doesn't use it? https://github.com/torvalds/linux/blob/6fea5fabd3323cd27b2ab5143263f37ff29550cb/net/unix/af_unix.c#L785 looks to me like it's at least stored in UDS sockets?