Fix problem with fetching images via Cloudflare

Fix problem with fetching images via Cloudflare

* lisp/url/url-http.el (url-http): Factor out url-interactive-p.
* lisp/url/url-auth.el (url-basic-auth):
(url-basic-auth):
(url-digest-prompt-creds): Use it to not query the user.

* lisp/url/url-queue.el (url-queue-start-retrieve): Don't send a
bogus empty Authorization header (bug#54246) -- this triggers
Cloudflare's anti-attack software.  Instead rely on
url-request-noninteractive.

* lisp/url/url-vars.el (url-interactive-p): New utility function.