Optionally check the newsrc.eld file's timestamp before saving it.

* gnus-start.el (gnus-save-newsrc-file-check-timestamp): New option to
check the newsrc.eld file's timestamp before saving it.
(gnus-save-newsrc-file): Use it, with a prompt when the newsrc.eld
timestamp has changed to be newer.
gnus-icalendar.el: Support vcal format timezones

Old version only supported emacs known timezone.
Here we reuse icalendar code to extract timezone definition from
vcalendar format.

* gnus-icalendar.el: Support vcal format timezones
(gnus-icalendar-event--decode-datefield): use icalendar functions to
compute dates with associated timezone
(gnus-icalendar-event-from-ical): compute all timezones
Make signature respect format=flowed

* lisp/message.el (message-insert-signature): Use `newline' instead of
inserting explicit "\n".
lisp/ChangeLog: Merge from emacs-24; up to r117596 (Emacs trunk)
lisp/ChangeLog: Merge from emacs-24