The URL parser function can't reject a bad IPv6 address properly when
curl was built without IPv6 support.
Reported-by: Marcel Raad
Fixes#4556Closes#4572
... to make the host name "usable". Store the scope id and put it back
when extracting a URL out of it.
Also makes curl_url_set() syntax check CURLUPART_HOST.
Fixes#3817Closes#3822
In order for this API to fully work for libcurl itself, it now offers a
CURLU_GUESS_SCHEME flag that makes it "guess" scheme based on the host
name prefix just like libcurl always did. If there's no known prefix, it
will guess "http://".
Separately, it relaxes the check of the host name so that IDN host names
can be passed in as well.
Both these changes are necessary for libcurl itself to use this API.
Assisted-by: Daniel Gustafsson
Closes#3018
