Add basic getsockopt shim for reading SO_ERROR

[WhySoBad]

Hi,

This pull request adds the getsockopt shim. Currently, it only supports reading the SO_ERROR option.

By this, Miri is now able to run programs which use tokio/mio sockets. Thus, I also added a simple tokio socket test which tests connecting/accepting and reading/writing. Let me know if I should test more things with the tokio sockets.
I've also ran the tokio TCP test suite and we pass everything except:

• Some tests which are using syscalls which aren't shimmed (mainly socket options)
• One test failing due to a Miri panic when the write end of a connecting socket is closed before it received a writable event
• One test failing due to Miri not clearing the readiness after a short read/write on a linux target

For the latter two points I'll soon open pull requests here.

[rustbot]

Thank you for contributing to Miri! A reviewer will take a look at your PR, typically within a week or two.
Please remember to not force-push to the PR branch except when you need to rebase due to a conflict or when the reviewer asks you for it.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[RalfJung]

Amazing how everything is coming together and now we can actually run that Tokio smoke test. :)

I still have some comments though, of course. ;)
@rustbot author

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[rustbot]

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

[WhySoBad]

@rustbot ready

[RalfJung]

That truncation of the value is really annoying, we may have to iterate a bit until we find a good way to deal with that.
@rustbot author

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[WhySoBad]

I think if you restart the failed CI job then everything should be fine.

@rustbot ready

[RalfJung]

Very nice. :)

Just one last question. Please also squash the commits.
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[WhySoBad]

@rustbot ready