uucore: replace unsafe mkfifo wrapper with nix crate implementation

[mattsu2020]

Replace the unsafe libc::mkfifo wrapper with a safe implementation using the nix crate. This change improves code safety by eliminating unsafe code blocks and leveraging the well-tested nix crate for system calls. The functionality remains identical - creating named pipes with 0o666 permissions - but now uses a safer, more idiomatic Rust approach.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (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/csplit/csplit-heap is now passing!
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!

[cakebaker]

I don't know if you have seen it: the redox job in the CI currently fails with:

error[E0425]: cannot find function `mkfifo` in module `nix::unistd`
   --> src/uucore/src/lib/features/fs.rs:840:18
    |
840 |     nix::unistd::mkfifo(path, nix::sys::stat::Mode::from_bits_truncate(0o666))?;
    |                  ^^^^^^ not found in `nix::unistd`
    |
note: found an item that was configured out
   --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nix-0.30.1/src/unistd.rs:808:8
    |
807 | #[cfg(not(target_os = "redox"))] // RedoxFS does not support fifo yet
    |          --------------------- the item is gated here

[xtqqczze]

We should open a nix issue, it seems redoxos does in fact support mkfifo:

https://github.com/redox-os/relibc/blob/9340d76a92fa1c454ae0136d9d7d7f3a41bf4788/src/header/sys_stat/mod.rs#L190

[xtqqczze]

I think we could remove the make_fifo wrapper and just use mkfifo(path, Mode::from_bits_truncate(0o666)) everywhere?

[cakebaker]

@xtqqczze I opened a PR there to remove the gate: nix-rust/nix#2749

[cakebaker]

I think we could remove the make_fifo wrapper and just use mkfifo(path, Mode::from_bits_truncate(0o666)) everywhere?

Yes, that's a possibility.