clippy: fix nightly map_unwrap_or lint

[xtqqczze]

https://rust-lang.github.io/rust-clippy/master/index.html#map_unwrap_or

Plus some stylistic tweaks

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will not alter performance

✅ 142 untouched benchmarks
⏩ 180 skipped benchmarks¹

---

_{Comparing xtqqczze:clippy/map_unwrap_or (d10b837) with main (42b2ad8)}

Footnotes

1. 180 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. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[xtqqczze]

    failures:

    ---- test_dd::test_final_stats_three_char_limit stdout ----
    ---- test_dd::test_final_stats_three_char_limit stderr ----


    failures:
        test_dd::test_final_stats_three_char_limit

    test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 4720 filtered out; finished in 0.25s
    
  stderr ───

    thread 'main' (23632) panicked at tests/uutests/src/lib/util.rs:412:35:
    called `Option::unwrap()` on a `None` value

• lib::CmdResult::code() called Option::unwrap() on a None value #10455