Support peeking on unix sockets

[lucacasonato]

Is your feature request related to a problem?

Tokio gives the user the ability to peek at data on a TCP socket using TcpStream::peek (same on the split half). Under the hood this calls recv(2) with the MSG_PEEK flag set. This feature is not available for unix sockets, even they support this feature too AFAIK.

Describe the solution you'd like

UnixStream::peek that behaves the same way as TcpStream::peek.

Describe alternatives you've considered

UnixStream::read with a temporary buffer. This is not as versatile or as easy to use though.

Additional info

This will need support in mio: tokio-rs/mio#1418