more: replace nix by rustix

[oech3]

I noticed that write is incorrectly used instead of write_all which might not write everything. But a large refactoring is needed to fix it...

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (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)

[xtqqczze]

We have OwnedFd, so can do File::from(fd) without unsafe.

[oech3]

But it is required at many places.

[xtqqczze]

Another reason to use standard library instead of rustix is to get clippy warnings:

warning: written amount is not handled
  --> tests/by-util/test_more.rs:51:5
   |
51 |     controller.write(b"q").unwrap();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: use `Write::write_all` instead, or handle partial writes
   = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.95.0/index.html#unused_io_amount
   = note: `-W clippy::unused-io-amount` implied by `-W clippy::all`
   = help: to override `-W clippy::all` add `#[allow(clippy::unused_io_amount)]`

[oech3]

But it requires many diff. So I don't at here.

[xtqqczze]

I'd rather remove both nix and rustix. Close this PR, and I'll fix it.