df: replace unsafe libc sync with nix crate

[mattsu2020]

Summary

df currently uses an unsafe libc::sync() call for --sync, with a separate Android-specific syscall(SYS_sync) branch.

This PR replaces that code with nix::unistd::sync(). That removes the manual libc/syscall handling and keeps the implementation simpler.

Changes

• replace the unsafe sync call in src/uu/df/src/df.rs
• remove the Android-specific SYS_sync branch
• add nix as a Unix-only dependency for uu_df

Testing

• cargo check -p uu_df
• cargo test -p uu_df

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (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)
Congrats! The gnu test tests/rm/isatty is no longer failing!
Congrats! The gnu test tests/tail/retry is no longer failing!

[xtqqczze]

libc added sync support for android in v0.2.142 (rust-lang/libc@0ba1fc4), so this should be fine.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/cut-huge-range is now passing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/pr/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/cut-huge-range is now passing!

[xtqqczze]

still not a fan of AI descriptions