rc: add a backlight service to save/restore backlight levels

The default on my laptop is annoyingly bright, and this is a useful
feature to mitigate that. The backlight script is largely a copy of

rc: add a backlight service to save/restore backlight levels

The default on my laptop is annoyingly bright, and this is a useful
feature to mitigate that. The backlight script is largely a copy of the
mixer service which provides the same value for mixers, but this one is
specifically dependant on kld to allow DRM drivers a chance to attach.

Note that it's off by default to avoid interference with DEs, and
document the capability in backlight(8). Set backlight_enable=YES in
rc.conf(5) to enable save/restore.

Relnotes: maybe
Reviewed by: bapt, ivy, manu, ziaee
Differential Revision: https://reviews.freebsd.org/D59296

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