Reimplement nnimap, and do tweaks to the rest of the code to support that.

Reimplement nnimap, and do tweaks to the rest of the code to support that.

* gnus-int.el (gnus-finish-retrieve-group-infos)
(gnus-retrieve-group-data-early): New functions.

* gnus-range.el (gnus-range-nconcat): New function.

* gnus-start.el (gnus-get-unread-articles): Support early retrieval of
data.
(gnus-read-active-for-groups): Support finishing the early retrieval of
data.

* gnus-sum.el (gnus-summary-move-article): Pass the move-to group name
if the move is internal, so that nnimap can do fast internal moves.

* gnus.el (gnus-article-special-mark-lists): Add uid/active tuples, for
nnimap usage.

* nnimap.el: Rewritten.

* nnmail.el (nnmail-inhibit-default-split-group): New internal variable
to allow the mail splitting to not return a default group.  This is
useful for nnimap, which will leave unmatched mail in the inbox.

* utf7.el (utf7-encode): Autoload.