Add IOV_MAX and UIO_MAXIOV constants

[Thomasdezeeuw]

These constant can be used to determine the maximum number of iovecs can
be passed to functions like readv/writev.

Linux like uses UIO_MAXIOV, while the BSD family uses IOV_MAX.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) soon.

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[Thomasdezeeuw]

I don't know how to add tests for these constants. I also couldn't run the tests locally as it failed with a huge list of errors.

[JohnTitor]

Looks good, thanks!