Make fields in UnknownCmsg public

[hax0r31337]

What does this PR do

Made fields in nix::sys::socket::UnknownCmsg public.
This change will allow user to implement custom parsing for unsupported message, such as SOL_IP, IP_RECVORIGDSTADDR.

Checklist:

• I have read CONTRIBUTING.md
• A change log has been added if this PR modifies nix's API

[SteveLauC]

Hi, thanks for the PR!

SOL_IP, IP_RECVORIGDSTADDR

You mean the socket option? If so, it has already been supported by Nix in #1772, the option type is named Ipv4OrigDstAddr, I think it is more like a BSD style naming:

And the man page only mentions the latter, shorter name.

Linux man page uses IP_RECVORIGDSTADDR.

[hax0r31337]

I found out shortly after I opened this pull request.
But expose field of UnknownCmsg to the user might be useful.

[SteveLauC]

But expose field of UnknownCmsg to the user might be useful.

Make sense, left a comment.

[SteveLauC]

Looks good, thanks!