url-http-ntlm: Remove limit of one username and password per server
* url-http-ntlm.el: Remove comment about only supporting one
username and password. Do not make url-http-ntlm--last-args a
buffer-local variable.
(url-http-ntlm--auth-storage): Change docstring to not mention one
user and password limitation.
(url-http-ntlm--default-users): New variable.
(url-http-ntlm--ensure-user): New function.
(url-http-ntlm--get-stage): Take a url argument. Store a key in
url-http-ntlm--last-args.
(url-http-ntlm--authorisation): Take a realm argument. Use a key
when accessing url-http-ntlm--last-args.
(url-ntlm-auth): Ensure the received URL has its user slot set
before processing it.