Improve battery.el UPower support

Improve battery.el UPower support

For discussion, see the following threads:
https://lists.gnu.org/archive/html/emacs-devel/2020-01/msg00843.html
https://lists.gnu.org/archive/html/emacs-devel/2020-02/msg00042.html
https://lists.gnu.org/archive/html/emacs-devel/2020-02/msg00282.html

* etc/NEWS: Announce that battery-upower is enabled by default.

* lisp/battery.el (battery-upower-device): Accept both battery and
line power device names, or a list thereof (bug#39491).
(battery-upower-line-power-device): Remove user option; superseded
by battery-upower-device.
(battery-upower-subscribe): New user option.
(battery-status-function): Check whether a UPower service is
provided without activating it.
(display-battery-mode): Subscribe to UPower signals when using
battery-upower.
(battery-upower): Merge data from multiple power sources.  Calculate
terse battery status %b based on average battery load percentage
rather than coarse and often missing BatteryLevel (bug#39491).  Add
support for average temperature %d.

(battery-upower-dbus-service)
(battery-upower-dbus-interface)
(battery-upower-dbus-path)
(battery-upower-dbus-device-interface)
(battery-upower-dbus-device-path)
(battery-upower-device-all-properties): Rename to...
(battery-upower-service)
(battery-upower-interface)
(battery-upower-path)
(battery-upower-device-interface)
(battery-upower-device-path)
(battery--upower-device-properties): ...these, respectively.

(battery-upower-device-list): Rename to...
(battery--upower-devices) ...this.  Return a flat list of device
names determined by battery-upower-device.
(battery-upower-types, battery-upower-states)
(battery-upower-device-property, battery-upower-device-autodetect):
Remove.
(battery--upower-signals): New variable.
(battery--upower-signal-handler, battery--upower-props-changed)
(battery--upower-unsubscribe, battery--upower-subsribe)
(battery--upower-state): New functions.

* test/lisp/battery-tests.el (battery-upower-state)
(battery-upower-state-unknown): New tests.