curl_path: make sure just whitespace is illegal

curl_path: make sure just whitespace is illegal

This function could previously accidentally return true and a NULL path
if only whitespace was provided as argument.

Also, make it stricter and do not allow CR or LF within the string.

Use more strparse parsing.

Drop the comment saying this is from OpenSSH as it has now been
rewritten since then.

Closes #19141