uucore: replace unsafe libc calls with safe nix crate wrappers

[mattsu2020]

Replace direct unsafe libc function calls with safe nix crate wrappers for process management functions.
This improves code safety and maintainability by leveraging nix's type-safe abstractions for system calls like geteuid, getpgrp, getsid, and signal handling.
The changes maintain identical functionality while eliminating unsafe code blocks and providing better error handling.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/follow-name (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Congrats! The gnu test tests/expand/bounded-memory is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[oech3]

@mattsu2020 Are you inrerested in replacing nix and libc deps with rustix at least on Linux?

[mattsu2020]

@mattsu2020 Are you inrerested in replacing nix and libc deps with rustix at least on Linux?

I thought nix was the standard, but it seems rustix is also in use.
Since rustix is newer, replacing it with that seems like a good idea.

[oech3]

I think rustix is wrapping libc at many case, but using syscall directry at Linux for perf.

[sylvestre]

having less unsafe is ideal :)