Implement conversion from std::net::Shutdown

[gnaaman-dn]

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What does this PR do

std::net::Shutdown and nix's Shutdown are basically the same. It is possible in theory to completely the nix version in favour of std's, but it lacks the Hash implementation, so I opted to just add a conversion.

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