Add Sctp protocol and MSG_NOTIFICATION

[gnaaman-dn]

What does this PR do

• Adds the Sctp variant to the SockProtocol enum, allowing the user to create SCTP sockets with either SockType::Stream or SockType::SeqPacket.
• (Linux Only) Expose MSG_NOTIFICATION through MsgFlags so that it won't be truncated when converting from i32.

IPPROTO_SCTP is defined in RFC-6458, but it is not supported by all OSs.
I've cfg'd-out targets that failed in CI.

MSG_NOTIFICATION is Linux-specific, as far as I can tell, but is not exposed through Android's libc, so I've limited it to linux and not linux_android.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

Thanks for the PR, just 2 cfg nits:)

[SteveLauC]

Thanks!

[gnaaman-dn]

Thank you @SteveLauC !