Add network socket shutdown shim

[WhySoBad]

Hi,

This pull request adds the shim for the shutdown syscall for TCP sockets.
I've ran the tokio TCP tests using Miri and noticed that many of them require TcpStream::shutdown so it would be nice to also have this shimmed.

[rustbot]

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[WhySoBad]

The CI fails for the same reason mentioned here: #4984 (comment).
It's because WSAESHUTDOWN is not a known error kind in the standard library.

What would you suggest doing to handle such unknown I/O errors?

[RalfJung]

I didn't even know this operation exists.^^ I wonder what the point is, why wouldn't you just close?

Anyway, this mostly looks good, except for some test nits.
@rustbot author

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[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[WhySoBad]

@rustbot ready

[RalfJung]

Just one final nit. :) Please also squash the commits.

@rustbot author

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[WhySoBad]

@rustbot ready