paste: accept a hyphen-leading -d delimiter list

[arbelonson-source]

What

-d's value was rejected as an unrecognized flag when given as its
own argument:

$ paste -d -x f1 f2

# GNU: accepts it, uses '-' as the delimiter (first char of the cycle)
a-1
b-2

# uutils, before this PR:
error: unexpected argument '-x' found
  tip: to pass '-x' as a value, use '-- -x'

The attached forms (-d-x, -d=-x) already worked; only the
separate-argument form broke -- the same allow_hyphen_values gap
already fixed this session for several other options (sort --parallel, nl's numeric options, ptx --gap-size/--width, join -t, csplit --suffix-format).

Testing

• cargo test -p uu_paste / full tests/by-util/test_paste.rs suite: 21 passed, 0 failed.
• Added a regression test for the separate-argument hyphen-leading case.
• Manually diffed -d -x, -d ,, -d -, -d -- against GNU paste 9.11 under LC_ALL=C.
• cargo clippy -p uu_paste --all-targets -- -D warnings and cargo fmt --check: clean.

---

This PR was written with AI assistance (Claude Opus 5, via Claude Code). I've tested the changes but please review the code carefully.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (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)
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[cakebaker]

Thanks!