kill: replace nix::sys::signal with rustix::process

[mattsu2020]

Summary

• Replace nix::sys::signal usage in the kill utility with rustix::process APIs as part of the migration away from nix (see chore(deps): update rust crate clap to v4.5.30 #7317)
• Use rustix's explicit per-case APIs:
  • kill_process for positive PIDs
  • kill_process_group for negative PIDs
  • kill_current_process_group for PID 0
  • test_kill_* variants for signal 0
• Keep a small raw libc::kill path for libc realtime signals (SIGRTMIN..=SIGRTMAX on Linux/Android), because rustix's Signal contract does not allow libc-reserved realtime signal values to be used for sending signals
• Guard against i32::MIN overflow with checked_neg() when negating negative PIDs
• Use safe Pid::from_raw() with expect() instead of Pid::from_raw_unchecked

Tests

• cargo build -p uu_kill
• cargo test -p coreutils --test tests --features kill -- test_kill_ (37 passed on macOS)

Note: the realtime signal send test is Linux/Android-only and is therefore not run on macOS.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout-group (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)

[codspeed-hq]

Merging this PR will not alter performance

✅ 268 untouched benchmarks
⏩ 97 skipped benchmarks¹

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_{Comparing mattsu2020:kill_rustix (e6ac11c) with main (8e4c21b)}

Footnotes

1. 97 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩