Fix potential inifinite loop reading file content with `Curl_get_line()`
when a filename passed via these options are pointing to a directory
entry (on non-Windows):
- `--alt-svc` / `CURLOPT_ALTSVC`
- `-b` / `--cookie` / `CURLOPT_COOKIEFILE`
- `--hsts` / `CURLOPT_HSTS`
- `--netrc-file` / `CURLOPT_NETRC_FILE`
Fix by checking for this condition and silently skipping such filename
without attempting to read content. Add test 1713 to verify.
Mention in cookie documentation as an accepted case, also show a verbose
message when a directory is detected. Extend test 46 to verify if such
failure lets the logic continue to the next cookie file.
Reported-and-based-on-patch-by: Richard Tollerton
Fixes#20823Closes#20826 (originally-based-on)
Follow-up to 769ccb4d42#19140Closes#20873
