Add IPPROTO_IP variant to SockProtocol

[evanrittenhouse]

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What does this PR do

Adds the new IPPROTO_IP variant to SockProtocol as described in #1953

Checklist:

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

[evanrittenhouse]

I'm not quite sure how to handle the conflict between libc::NETLINK_ROUTE and libc::IPPROTO_IP, both of which are defined as 0.

@asomers, I've seen you around in quite a few issues. Do you have any preference? Sorry if this is a basic question, this is my first PR to the project. Excited to learn more

[asomers]

To define two enum values that have the same representation, create a constant instead. See SockProtocol::CanRaw for an example.