Fix new issues found by `proselint`.
Also:
- silence this technical warning:
```
:0: DeprecationWarning: /home/runner/.proselintrc was found instead of a JSON file. Rename to /home/runner/.proselintrc.json.
```
- fix an input filename.
`proselints` fails now if an input file is missing.
Reported-by: Jay Satiro
Bug: https://github.com/curl/curl/pull/15291#issuecomment-2410505100Closes#15293
`Curl_bufq_unwrite()` used the head instead of the tail chunk to shrink
the bufq's content. Fix this and add test case that checks correct
behaviour.
Amended test 2601 accordingly.
Reported-by: Chris Stubbs
Closes#15136
