join: accept a hyphen-leading -t separator, quote it in the error

[arbelonson-source]

What

-t's value was rejected as an unrecognized flag when given as its
own argument:

$ join -t -x f1 f2

# GNU
join: multi-character tab '-x'

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

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

Also fixed

The resulting "multi-character tab" message doesn't quote the value,
a separate, pre-existing bug in the same code path (reachable before
this fix too, just via a non-hyphen multi-character value like -t ab, not only the hyphen-leading one this PR is really about):

# GNU
join: multi-character tab 'ab'

# uutils, before this PR
join: multi-character tab ab

Fixed since this fix's own regression test for the hyphen-leading
case would otherwise still fail against it -- a hyphen-leading
multi-character value reaches the exact same message construction as
any other one.

Testing

• cargo test -p uu_join / full tests/by-util/test_join.rs suite: 46 passed, 0 failed.
• Fixed the existing tab_multi_character test, which had encoded the unquoted wording as the expected result.
• Added a regression test for the separate-argument hyphen-leading case (the actual reported bug).
• Manually diffed -t -x, -t ab, -t :, -t - against GNU join 9.11 under LC_ALL=C.
• cargo clippy -p uu_join --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/date/resolution (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)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)