csplit: accept a hyphen-leading --suffix-format value

[arbelonson-source]

What

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

$ csplit --suffix-format -%02d f /pat/

# GNU: accepts it, splits normally with files named xx-00, xx-01, ...
# uutils, before this PR:
error: unexpected argument '-%' found
  tip: to pass '-%' as a value, use '-- -%'

The attached forms (-b-%02d, --suffix-format=-%02d) 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).

Testing

• cargo test -p uu_csplit / full tests/by-util/test_csplit.rs suite: 93 passed, 0 failed.
• Added a regression test for the separate-argument hyphen-leading case, checking both stdout and the actual split filenames produced.
• Manually diffed --suffix-format -%02d and --suffix-format=z%02d (both output and the resulting filenames) against GNU csplit 9.11 under LC_ALL=C.
• cargo clippy -p uu_csplit --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:

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!