Various battery.el improvements (bug#41808)

Various battery.el improvements (bug#41808)

* lisp/battery.el: Mention BSD support in Commentary.  Don't load
preloaded lisp/emacs-lisp/timer.el.
(battery--files): New function.
(battery--find-linux-sysfs-batteries): Use it and make fewer
syscalls.
(battery-status-function): Perform GNU/Linux checks in increasing
order of obsolescence: sysfs, ACPI, and then APM.  Simplify Darwin
check.  Add :version tag now that battery-upower is the default.
(battery-echo-area-format, battery-mode-line-format): Mention %s.
(battery-load-low, battery-load-critical): New faces.
(battery-update): Display battery-mode-line-format even if
percentage is N/A.  Apply faces battery-load-low or
battery-load-critical according to the percentage, but append them
so they don't override user customizations.  Update all mode lines
since we are in global-mode-string.
(battery-linux-proc-apm-regexp): Mark as obsolete, replacing with...
(battery--linux-proc-apm): ...this new rx definition.
(battery-linux-proc-apm): Use it.  Fix indentation.  Simplify.
(battery--acpi-rate, battery--acpi-capacity): New rx definitions.
(battery-linux-proc-acpi): Use them.  Fix pathological whitespace
regexps.  Simplify.
(battery-linux-sysfs): Fix docstring and indentation.  Reduce number
of file searches.  Simplify.
(battery-bsd-apm): Fix docstring.  Simplify.
(battery-pmset): Fix docstring.  Simplify ID regexp.

* lisp/emacs-lisp/rx.el (rx-define): Indent as a defun.

* test/lisp/battery-tests.el (battery-linux-proc-apm-regexp): Test
new battery--linux-proc-apm rx definition.
(battery-acpi-rate-regexp, battery-acpi-capacity-regexp): New tests.