clippy: fix clippy warnings

[sylvestre]

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

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[cakebaker]

Hm, is this PR supposed to fix all warnings from the map_unwrap_or lint? When I run cargo clippy -- -W clippy::map_unwrap_or on the main branch I get warnings in utils like ls and sort and there are no changes to those utils in this PR.

[sylvestre]

it fixed the issues that I saw :(
We can fix the rest in a different PR

[cakebaker]

Looks good, though I would move the changes made to split and tac to its own commit because they are unrelated to the map_unwrap_or lint. They seem to fix warnings from the needless_as_bytes lint.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/usage_vs_getopt. tests/misc/usage_vs_getopt is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/basenc/basenc is no longer failing!
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)