* lisp/gnus/gnus-dbus.el (gnus-dbus-register-sleep-signal): Apparently
this needs to be :system -- perhaps because PrepareForSleep is a
system-level event?
* lisp/gnus/gnus-dbus.el: New library, registering a signal that
closes all Gnus servers when the system is going to sleep.
* lisp/gnus/gnus-start.el: Check new option
`gnus-dbus-close-on-sleep', and register the appropriate D-Bus signal
if it is non-nil.
* lisp/gnus/gnus.el: New gnus-dbus customization group.
* doc/misc/gnus.texi: Document.
