Enable illumos support

[Toasterson]

I had this around for a while in my tools it works well for me on illumos. Happy to have it upstreamed.

[coveralls]

Pull Request Test Coverage Report for Build 6362496496

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-1.0%) to 84.04%

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Files with Coverage Reduction	New Missed Lines	%
../../../.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-channel-0.3.28/src/mpsc/mod.rs	1	89.58%
../../../.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-channel-0.3.28/src/mpsc/sink_impl.rs	3	76.92%

Totals
Change from base Build 6147264426:	-1.0%
Covered Lines:	337
Relevant Lines:	401

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💛 - Coveralls