Register an HTTP authentication method.
TYPE is a string or symbol specifying the name of the method.
This should be the same thing you expect to get returned in
an Authenticate header in HTTP/1.0 - it will be downcased.
FUNCTION is the function to call to get the authorization information.
This defaults to `url-?-auth', where ? is TYPE.
RATING a rating between 1 and 10 of the strength of the authentication.
This is used when asking for the best authentication for a specific
URL. The item with the highest rating is returned.
(fn TYPE &optional FUNCTION RATING)