Use new auth-source search API for mail-source, nnimap, nntp.

Use new auth-source search API for mail-source, nnimap, nntp.

* nntp.el: Autoload `auth-source-search'.
(nntp-send-authinfo): Use it.  Note TODO.

* nnimap.el: Autoload `auth-source-search'.
(nnimap-credentials): Use it.
(nnimap-open-connection-1): Ask for the virtual server and physical
address in one shot.

* mail-source.el: Autoload `auth-source-search'.
(mail-source-keyword-map): Note order matters.
(mail-source-set-1): Get all the mail-source source values and
defaults and search auth-source on those if needed.  This can all
probably be simplified.