illumos: expose PIPE_BUF constant

[dancrossnyc]

Expose the PIPE_BUF constant on Solaris/illumos. We have verified that these values are the same between the two variants of the system.

[rustbot]

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) some time within the next two weeks.

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[rustbot]

Some changes occurred in solarish module

cc @jclulow, @pfmooney

[jclulow]

What are you planning to use this constant for?

[labbott]

What are you planning to use this constant for?

This is needed to get tokio-pipe to build https://github.com/yskszk63/tokio-pipe/blob/master/src/lib.rs#L40

[jclulow]

Alright that makes sense, thanks.

[dancrossnyc]

Ping? (@JohnTitor)

[JohnTitor]

Sorry for the delay, thanks for the PR!

[dancrossnyc]

Thank you!