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
That is functions that are declared global but are not used from outside
of the file in which it is declared. Such functions should be made
static or even at times be removed.
It also verifies that all used curl_ prefixed functions are "blessed"
Closes#3538
