Function: url-get-authentication

Return an authorization string suitable for use in the WWW-Authenticate
header in an HTTP/1.0 request.

URL is the url you are requesting authorization to. This can be either a
string representing the URL, or the parsed representation returned by
`url-generic-parse-url'
REALM is the realm at a specific site we are looking for. This should be a
string specifying the exact realm, or nil or the symbol 'any' to
specify that the filename portion of the URL should be used as the
realm
TYPE is the type of authentication to be returned. This is either a string
representing the type (basic, digest, etc), or nil or the symbol 'any'
to specify that any authentication is acceptable. If requesting 'any'
the strongest matching authentication will be returned. If this is
wrong, it's no big deal, the error from the server will specify exactly
what type of auth to use
PROMPT is boolean - specifies whether to ask the user for a username/password
if one cannot be found in the cache (fn URL REALM TYPE PROMPT &optional ARGS)