shuf: treat -e as a flag, not as a multi-value arg

[BenWiederhake]

This PR implements and tests support for the weird permitted argument order of shuf, especially for invocations like shuf -ez firstarg secondarg, where the z shall not be interpreted as a value to -e.

This moves us closer to passing the GNU testsuite.

Related:

• Discussed in New argument parser #4254 (comment) (turns out, I was wrong, it is possible)
• Relates to implement proof of concept for shuf echo args / file uutils-args#104 (which is kinda the same patch, but using the new uutils-arg).

This PR sadly slightly degrades the quality of the error message in one case:

Before:

$ cargo run -- a b
error: unexpected argument 'b' found

Usage: /path/to/target/debug/shuf [OPTION]... [FILE]
       /path/to/target/debug/shuf -e [OPTION]... [ARG]...
       /path/to/target/debug/shuf -i LO-HI [OPTION]...

For more information, try '--help'.
[$? = 1]

After:

$ cargo run -- a b
/path/to/target/debug/shuf: unexpected argument 'b' found
Try '/path/to/target/debug/shuf --help' for more information.
[$? = 1]

Note that this does not affect the --help message, so I think it's good enough.

[tertsdiepraam]

Great! Thanks!

[BenWiederhake]

Changes since last push:

• Rebased on newest main
• Applied two cleanups – thanks @tertsdiepraam!

[BenWiederhake]

Changes since last push:

• Fix two lints that aren't in pre-commit, but were caught by CI (specifically: prefer .cloned(), and drop an unnecessary .to_string())

[BenWiederhake]

Changes since last push:

• Rebased on current main
• Added two new tests for error inputs, specifically multiple files (test_shuf_two_input_files and test_shuf_three_input_files). Thanks @cakebaker!
• Added a new test for a single dash as filename. (test_explicit_stdin_file)

[BenWiederhake]

Both failures seem to be flakes:

• test_split::test_round_robin_limited_file_descriptors (split: unable to open 'xav'; aborting)
• test_timeout::test_kill_subprocess (panicked at ''' does not contain 'xyz'')

[tertsdiepraam]

Thanks! Your updates on the force pushes are great by the way. Really useful while reviewing!