Use accept4 where possible

[Thomasdezeeuw]

In sys::TcpListener::accept.

[Thomasdezeeuw]

I plan to work on this after I've converted TcpListener to use IoSource.

[Thomasdezeeuw]

Doing this after #1242 is merged.

[Thomasdezeeuw]

Note to self: on macOS accept4(2) doesn't exists, but we might not have to set the TcpStream to non-blocking: #1242 (comment).