curl_get_line: fix potential infinite loop when filename is a directory

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 #20823
Closes #20826 (originally-based-on)
Follow-up to 769ccb4d42 #19140

Closes #20873

docs/cmdline-opts/cookie.md

@@ -54,7 +54,8 @@ the Netscape format.
 
 Users often want to both read cookies from a file and write updated cookies
 back to a file, so using both --cookie and --cookie-jar in the same command
-line is common.
+line is common. curl ignores filenames specified with --cookie which do not
+exist or point to a directory.
 
 If curl is built with PSL (**Public Suffix List**) support, it detects and
 discards cookies that are specified for such suffix domains that should not be

docs/libcurl/opts/CURLOPT_COOKIEFILE.md

@@ -58,6 +58,8 @@ list of files to read cookies from.
 The cookies are loaded from the specified file(s) when the transfer starts,
 not when this option is set.
 
+libcurl ignores filenames which do not exist or point to a directory.
+
 # SECURITY CONCERNS
 
 This document previously mentioned how specifying a non-existing file can also