cygwin: Add missing utmp/x.h, grp.h, and stdio.h interfaces

[500-internal-server-error]

Description

These constants and functions are required for building uutils-coreutils for Cygwin.

I'm not sure what the logic is on where things are placed, I just made it work. Additionally, I can't figure out how the tests work to fix it, but it's functional enough to unblock my immediate needs. Hopefully someone can give me guidance on how to clean up the changes and fix the tests?

Sources

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/winsup/cygwin/include/cygwin/config.h#L50
https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/newlib/libc/include/stdio.h#L138-L139

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/winsup/cygwin/include/cygwin/utmp.h#L20-L33

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/winsup/cygwin/include/utmpx.h#L21-L32

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/newlib/libc/include/stdio.h#L120-L122

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/newlib/libc/include/grp.h#L64-L65

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/winsup/cygwin/include/utmpx.h#L21-L32

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/winsup/cygwin/include/utmpx.h#L42-L49

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 label +stable-nominated

[500-internal-server-error]

I don't think I touched MacOS, so I'm going to assume the fail is not due to my changes. I don't have hardware to test locally either...

[tgross35]

The changes themselves LGTM, thank you for including links. A request for history though - could you squash the last two commits and make the commit message something like "cygwin: Add missing utmp/x.h, grp.h, and stdio.h interfaces"? We use cherry picks here so it's nice if the log is a bit more specific than "cygwin stuff" :)

(Feel free to just paste the great PR description/sources you have into the commit message, that would be awesome. We don't squash+merge because you can't choose the strategy with merge queues.)

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[tgross35]

Cc also platform maintainer @Berrysoft

[Berrysoft]

I have checked the changes and verified that they match the cygwin headers.

[500-internal-server-error]

@tgross35 I have squashed the last 2 commits as requested.

[500-internal-server-error]

@rustbot ready