* mail-source.el: Load auth-source.el.

* mail-source.el: Load auth-source.el.
(mail-source-bind): Add comments.  Call auth-source-user-or-password to
get user name or password, if auth-sources is set up.

* gnus-registry.el (gnus-registry-split-strategy): New variable for
strategy of splitting with parent.
(gnus-registry-split-fancy-with-parent)
(gnus-registry-post-process-groups): Use it and fix prior
bug (returning a list as the split result).

* auth-source.el (auth-sources): Remove server parameter.
(auth-source-pick, auth-source-user-or-password)
(auth-source-user-or-password-imap)
(auth-source-user-or-password-pop3, auth-source-user-or-password-ssh)
(auth-source-user-or-password-sftp)
(auth-source-user-or-password-smtp): Remove server parameter.