Enable IP_BOUND_IF on illumos and Solaris

[hawkw]

The IP_BOUND_IF socket option, which is wrapped by the Socket::bind_device_by_index_{v4,v6} and
Socket::device_index_{v4,v6} is available on SunOS-like systems, such as illumos and Solaris, as well as macOS-like systems. However, these APIs are currently cfg-flagged to only be available on macOS-like systems.

This commit changes the cfg attributes to also enable these APIs on illumos and Solaris.

Fixes #560

This is a backport of PR #561 to the v0.5.x branch.

[Thomasdezeeuw]

Thanks @hawkw

[hawkw]

Ah, whoops --- it looks like I accidentally opened this PR against master rather than v0.5.x, although it was based on v0.5.x in my fork. My bad! There are now two copies of this commit on master (one of which is a 0-line diff) and none on v0.5.x:

I've opened a new PR #566 which is actually based on v0.5.x the way it should be. You may also want to rewrite history to remove the empty second commit to master (33291e2) though I'm not sure how you feel about that sort of thing. Sorry for the inconvenience!

[Thomasdezeeuw]

Why does GitHub even allow that? 🤦

[hawkw]

I think that GitHub will attempt to prevent you from accidentally opening a PR for a commit that was already merged, but since the source branch was based on the v0.5.x branch on my fork, it may have understood this PR as a totally separate, new commit, rather than the same commit back for a second time? I'm not sure, though. Oh well.