Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

message.el (message-point-in-header-p): Tweak the function to default to saying that we're not in the headers if there is no separator at all.  This makes it possible to use the Message version of `M-q' in buffers with no headers (bug#7987).
 (message-point-in-header-p): Fix last checkin to work with an empty mail-header-separator, too.
auth-source.el (auth-source-netrc-saver): If the user says "don't ask again, save the choice via customize.
message.el (message-send-mail-function): Add `sendmail-query-once'.
nnimap.el (nnimap-finish-retrieve-group-infos): If the server has ended the connection, bail out before waiting infinitely on a new connection.