xargs: reject quotes spanning input lines

[wtcpython]

Fixes #776.

The whitespace-delimited argument reader allowed single and double quotes to span input lines. Quotes opened on one line could be closed on a later line, so malformed input was combined into arguments instead of rejected.

Reject a newline while a quote is open, matching GNU and BSD xargs behavior. Add regression coverage for both quote types when reading through --arg-file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.08%. Comparing base (2a3eac9) to head (c564172).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #797      +/-   ##
==========================================
- Coverage   92.15%   92.08%   -0.07%     
==========================================
  Files          35       35              
  Lines        7377     7382       +5     
  Branches      383      383              
==========================================
  Hits         6798     6798              
- Misses        438      442       +4     
- Partials      141      142       +1     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing wtcpython:fix/xargs-reject-multiline-quotes (c564172) with main (2a3eac9)}

[github-actions]

Commit 37bf474 has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 314 / PASSED: 266 / FAILED: 42 / SKIPPED: 6
  Reference: TOTAL: 314 / PASSED: 267 / FAILED: 41 / SKIPPED: 6

Changes from main branch:
  TOTAL: +0
  PASSED: -1
  FAILED: +1

New test failures (1):
  - gnu/okdir_path_relative

[github-actions]

Commit c564172 has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 317 / PASSED: 274 / FAILED: 37 / SKIPPED: 6
  Reference: TOTAL: 313 / PASSED: 267 / FAILED: 40 / SKIPPED: 6

Changes from main branch:
  TOTAL: +4
  PASSED: +7
  FAILED: -3

Test improvements (3):
  + gnu/execdir_path_dot
  + gnu/execdir_path_empty
  + gnu/execdir_path_relative