[WIP] cookie: curl_filter_cookies public function

[jay]

[WIP] Expose a function to show a list of cookies for a url.

Ref: https://curl.haxx.se/mail/lib-2019-09/0100.html

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Filters a linked list of cookies to determine which match the URL. Untested. I already see a few mistakes.

/cc @dhoelzl

[bagder]

1. I think that if/when we provide a dedicated cookie match function, it should return the list of matching cookies in a better format than string in netscape format. It is a bit too error prone for users to have to parse that. I think there should be a struct curl_cookie for each cookie to allow the application to get the already parsed details from them.
2. I don't think the function needs a list of input cookies to set. It just complicates the function.
3. I think this PR should also provide a basic description of how the function works to make it easier to review and discuss. Possibly done as a wiki page.

[bagder]

I think the function name should start with curl_cookie_, to group nicer if/when we figure out other cookie-related functions to add.

[jay]

Sure, this is to get the ball rolling so that we have a topic. curl_filter_cookies (now curl_cookie_filter) takes cookies obtained with CURLINFO_COOKIELIST as input, filters that input according to url and the filtered list is received as output.

[jay]

I'm mothballing this due to lack of interest.