Add curl_url_strerror() to convert CURLUcode into readable string and
facilitate easier troubleshooting in programs using URL API.
Extend CURLUcode with CURLU_LAST for iteration in unit tests.
Update man pages with a mention of new function.
Update example code and tests with new functionality where it fits.
Closes#7605
configure --enable-debug now enables -Wassign-enum with clang,
identifying several enum "abuses" also fixed.
Reported-by: Gisle Vanem
Bug: 879007f811 (commitcomment-42087553)Closes#5929