feature request: support macOS's launch_activate_socket

[liudongmiao]

In shadowsocks-libev, there is launch_activate_socket in shadowsocks/shadowsocks-libev#1003.
In shadowsocks-rust, there is no such feature.

So, it's possible for shadowsocks-rust to support this feature?

There is another rust project https://github.com/LinkTed/doh-client ships with launch_activate_socket.

[zonyitoo]

It seems that you only create the TCP socket via launch_activate_socket, how about the UDP socket?

[zonyitoo]

https://github.com/LinkTed/doh-client/blob/master/src/listen/activation_socket.rs#L28-L34

It has memory leaks if launch_activate_socket returns cnt > 1.

[liudongmiao]

@zonyitoo I will try to implement and make a PR.