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

[mattsu2020]

Summary

• Replace nix dependency with rustix in the kill utility, continuing the ongoing effort to migrate away from nix (see chore(deps): update rust crate clap to v4.5.30 #7317)
• Use rustix's explicitly typed per-case kill APIs (kill_process, kill_process_group, kill_current_process_group) and their test_kill_* variants for signal 0 (EXIT), instead of nix's single signal::kill(pid, Option<Signal>) function
• Dispatch based on pid sign (>0 process, <0 process group, =0 current process group) and signal value (=0 uses test_kill_* variants)
• 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

Test plan

• All 33 existing kill integration tests pass (cargo test -p coreutils --test tests --features kill -- test_kill_)
• Build passes with no warnings (cargo build -p uu_kill)

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/tty-eof. tests/misc/tty-eof is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Congrats! The gnu test tests/pr/bounded-memory is now passing!

[codspeed-hq]

Merging this PR will improve performance by 13.71%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 2 improved benchmarks
✅ 315 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Memory	cp_recursive_deep_tree[(120, 4)]	699.2 KB	562.5 KB	+24.31%
⚡	Simulation	ls_recursive_balanced_tree[(6, 4, 15)]	50.9 ms	48.9 ms	+4.01%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing mattsu2020:kill_rustix (22377dc) with main (d09f351)}

Footnotes

1. 46 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. ↩