Cygwin: Add PTY and group API

[Berrysoft]

Description

Some methods required by nix crate.

Sources

• https://github.com/cygwin/cygwin/blob/a3863bfeb73f3c3832038685498c7c4148a06890/winsup/cygwin/include/pty.h#L11-L14
• https://github.com/cygwin/cygwin/blob/a3863bfeb73f3c3832038685498c7c4148a06890/winsup/cygwin/include/cygwin/grp.h#L19
• https://github.com/cygwin/cygwin/blob/a3863bfeb73f3c3832038685498c7c4148a06890/newlib/libc/include/grp.h#L67-L78

Checklist

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

[rustbot]

r? @tgross35

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[tgross35]

I see forkpty and openpty defined at https://github.com/cygwin/cygwin/blob/a3863bfeb73f3c3832038685498c7c4148a06890/winsup/cygwin/include/pty.h#L11-L14 and getgrouplist at https://github.com/cygwin/cygwin/blob/a3863bfeb73f3c3832038685498c7c4148a06890/winsup/cygwin/include/cygwin/grp.h#L19, but don't see getgrgid_r and the others in the directory you linked. Is https://github.com/cygwin/cygwin/blob/a3863bfeb73f3c3832038685498c7c4148a06890/newlib/libc/include/grp.h#L67-L78 the correct source?

Assuming so, lgtm. Please update the description / commit message to say what actually changed (Nix requires a lot of things)

[Berrysoft]

Yes, it's the correct source. I'll update the commit message later.

I tested nix by enabling --all-features.