Mostly based on recent language decisions from "everything curl":
- remove contractions (isn't => is not)
- *an* HTTP (consistency)
- runtime (no hyphen)
- backend (no hyphen)
- URL is uppercase
Closes#8646
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
