fn starts_with_plus

[oech3]

Lossy conversion is overhead just for checking that 1st byte == a valid unocode.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[sylvestre]

Please update the commit message for something more explicit. Thanks

[oech3]

updated