Initial implementation of HTTP Digest qop for url

Initial implementation of HTTP Digest qop for url

This also refactors digest authentication functions in url-auth.el.

* lisp/url/url-auth.el (url-digest-auth, url-digest-auth-create-key):
(url-digest-auth-build-response, url-digest-auth-directory-id-assoc):
(url-digest-auth-name-value-string, url-digest-auth-source-creds):
(url-digest-cached-key, url-digest-cache-key, url-digest-find-creds):
(url-digest-find-new-key, url-digest-prompt-creds): Add new functions
to simplify code and aid in unit testing.
(url-digest-auth-build-response): Hook up new functionality, or fall
back to previous.
(url-digest-auth-make-request-digest-qop):
(url-digest-auth-make-cnonce, url-digest-auth-nonce-count):
(url-digest-auth-name-value-string): Add new helper functions.
* test/lisp/url/url-auth-tests.el (url-auth-test-colonjoin):
(url-auth-test-digest-ha1, url-auth-test-digest-ha2):
(url-auth-test-digest-request-digest): Add a few tests as now more
features are testable via intermediate functions.
(url-auth-test-challenges, url-auth-test-digest-request-digest): Test
the new implementation.  Parts of these were accidentally already
merged in the past.