IMAP servers that allow anonymous authentication advertise that fact in
their capabilities as AUTH=ANONYMOUS.

nnimap currently lets you take advantage of this explicitly, by setting
(nnimap-authenticator anonymous) in the server definition. But this
requires you to know in advance that the server allows anonymous
authentication, and decide to use it.

Gnus could figure this out itself and anonymously log in, using the
attached patch. It seems to work fine, but there might be further
consequences I haven't thought of. And possibly we'd simply prefer that
settings like this be a little more explicit for the user?

Does anyone have any opinions on this?