openbsd: add devname(3)

[tobhe]

Description

I'm planning to add OpenBSD support to the drm crate, for that I need devname(3) to replace devname_r which is used on FreeBSD but not available on OpenBSD.

Sources

https://man.openbsd.org/devname.3
https://github.com/openbsd/src/blob/master/include/stdlib.h#L283
https://github.com/openbsd/src/blob/master/lib/libc/gen/devname.c#L73

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @tgross35 (or someone else) some time within the next two weeks.

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[rustbot]

Some changes occurred in OpenBSD module

cc @semarie

[tgross35]

It looks like this may be available on all the BSDs we support; OpenBSD (your link), NetBSD https://man.netbsd.org/devname.3, FreeBSD https://man.freebsd.org/cgi/man.cgi?query=devname&sektion=3&n=1, and Apple https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/devname.3.html. Would you be able to try moving this definition to unix/bsd/mod.rs so they all get it?