IPV6_RECVPKTINFO and IP_PKTINFO support

[mcginty]

This feature would include both new SockOpts and new ControlMessages to be used with sendmsg and recvmsg.

I have an unclean patch here: master...mcginty:ipv6-pktinfo, and will be happy to turn this into a nicer PR if the devs think it could get merged.

The only really gross part is that libc doesn't have the constants for IPV6_RECVPKTINFO or IP_PKTINFO, so I had to grab them out of the header files for macOS and Linux (the two platforms I tested on).

[asomers]

You really shouldn't be afraid to update libc. It's quite easy to get patches accepted there. Once you do that and fix portability issues, I think this would be a fine feature to add.

[mcginty]

@asomers sounds good. I'll keep this issue open since it's still a feature request for nix, and will work on a libc PR.

[mcginty]

pending merge of rust-lang/libc#980

[pusateri]

Merged in #990. Can be closed.