Improve subthread sorting.

* gnus-sum.el (gnus-sort-subthreads-recursive): New function.
(gnus-sort-threads-recursive): Use it.
(gnus-sort-threads): Unconditionally call `gnus-sort-threads-recursive'
again.  Now that determines how to sort subthreads.
Make subthread sorting customizable and add docs for it.

* gnus-sum.el (gnus-subthread-sort-functions): New defcustom.
(gnus-sort-threads-recursively): Delete defcustom.
(gnus-sort-threads-recursive): Adapt accordingly.

* gnus.texi (Sorting the Summary Buffer): Document new defcustom
`gnus-subthread-sort-functions' and remove the obsolete documentation
of `gnus-sort-threads-recursively'.
Add possibility to show the current thread's total score.

* gnus-sum.el (gnus-summary-make-menu-bar): Add "Current thread score"
menu entry.

* gnus-score.el (gnus-summary-current-score): Use prefix arg to show
the current thread's total score instead of the current article's
score.
* gnus-start.el (gnus-read-active-for-groups): Always mark the data as
dirty to ensure nnimap data being saved.
gnus-util.el (gnus-emacs-completing-read): Isolate XEmacs stuff
* nnrss.el (nnrss-discover-feed): Indent.
Dribble save cleanup

* gnus-start.el (gnus-dribble-save): Only save the dribble file if it's
not empty.
(mml2015-epg-key-image-to-string): Protect against bugging out on ttys.
Fix thread marking on dummy roots

* gnus-uu.el (gnus-uu-mark-thread, gnus-uu-unmark-thread): Work with
dummy roots, too.
* gnus.texi (Basic Usage): Mention that warp means jump here.
* gnus-int.el (gnus-warp-to-article): Mention that warp means jump.
* gnus-icalendar.el: New file.
Compilation warning fix

* gnus-icalendar.el (gnus-icalendar-event:inline-reply-buttons):
Include `handle' parameter.
* nnmbox.el (nnmbox-request-article): Don't change point.
Colorise output from commands so that it looks like it's on purpose

* mm-decode.el (mm-command-output): New face.
(mm-display-external): Use it.
* dgnushack.el (dgnushack-compile): Add a temporary check for
gnus-icalendar.
(The notmuch Engine): Mention notmuch.
(message-ignored-news-headers): Delete X-Gnus-Delayed before sending.
*** empty log message ***
rfc2047.el (rfc2047-encode-message-header): Unify charsets into a single one used for encoding the whole text in a header