find: print a trailing backslash in -printf

[SichenLiang]

A trailing backslash in a printf format was treated as the start of an escape, so reaching the end of the format returned Unexpected EOF.

Print the backslash literally and issue the same warning as GNU findutils. The parser is shared by -printf and -fprintf, so both now behave the same way. Add a regression test for the trailing-backslash case.

Fixes #880

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.27%. Comparing base (c1aeb11) to head (ccc43ff).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #881      +/-   ##
==========================================
+ Coverage   92.26%   92.27%   +0.01%     
==========================================
  Files          35       35              
  Lines        7564     7576      +12     
  Branches      392      393       +1     
==========================================
+ Hits         6979     6991      +12     
  Misses        443      443              
  Partials      142      142              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[codspeed]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing SichenLiang:printf-trailing-backslash (ccc43ff) with main (c1aeb11)}

[github-actions]

Commit ccc43ff has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 316 / PASSED: 273 / FAILED: 37 / SKIPPED: 6
  Reference: TOTAL: 317 / PASSED: 274 / FAILED: 37 / SKIPPED: 6

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

No result in this run (1) - hung, crashed, or renamed:
  ? posix/flag_comma (was PASS)

[sylvestre]

Thanks for your PR