Previously the code would just do that for the path when extracting the
full URL, which made a subsequent curl_url_get() of the path to
(unexpectedly) still return it without the leading path.
Amend lib1560 to verify this. Clarify the curl_url_set() docs about it.
Bug: https://curl.se/mail/lib-2023-06/0015.htmlCloses#11272
Reported-by: Pedro Henrique