Add recv_with_ancillary_data for udp socket

[yuguorui]

Add recv_with_ancillary_data for udp socket to facilitate IP_PKTINFO and IP_RECVORIGDSTADDR usage.

IP_RECVORIGDSTADDR is very handly for UDP transparent proxy with tproxy, and these patches are precondition for tokio to support ancillary data receiving.

struct cmsghdr parsing is not included in these patches on purpose, to avoid introducing addtional dependencies.

[Thomasdezeeuw]

Mio follows the API provided by the std lib, so this won't be accepted by Mio. However it's something I'm willing to consider for socket2: https://github.com/rust-lang/socket2.

[yuguorui]

Mio follows the API provided by the std lib, so this won't be accepted by Mio. However it's something I'm willing to consider for socket2: https://github.com/rust-lang/socket2.

Thanks for your reminding.
I understand that socket2 maybe a more advanced networking API, but it doesn't seem to be event driven, could socket2 be effectively driven by tokio?

[Thomasdezeeuw]

Thanks for your reminding. I understand that socket2 maybe a more advanced networking API, but it doesn't seem to be event driven, could socket2 be effectively driven by tokio?

You can use socket2::SockRef to call any method on socket2::Socket on mio::net::UdpSocket.