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

[kevans]

Summary

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.

Relnotes: maybe

[bapt]

libexec/rc/rc.d/backlight

4

this is weird, I bet this comes from copy/paste from rc.d/mixer, but you can probably right your own here (and use SPDX)

31

You probably want required kld not provide kld

[arrowd]

libexec/rc/rc.conf

45

Enabling this by default will clash with DEs doing their own backlight management.
But unless user call save manually, I think it should really get in the way.

libexec/rc/rc.d/backlight

73

I'd prefer /var/db/backlight/... or /var/db/backlight-${1}-state

[kevans]

libexec/rc/rc.conf

45

I'm cool with off-by-default, I'll just want to amend backlight(8) to mention that the cpability exists for th rest of us.

libexec/rc/rc.d/backlight

31

*sigh* EDOOFUS at the last minute

73

The naming here forces them to be /var/db/backlightN-state because I wanted to explicitly ignore aliases up in list_backlights. That said, I have no objection to /var/db/backlight and I've somewhat wished that mixer did the same.

[kevans]

Review feedback:

• Scope the backlight settings off into /var/db/backlight
• Turn it off by default, mention it in the manpage
• Condense the license/copyright header down and fix the PROVIDE/REQUIRE
• Use my preferred quoting style while we're here

[manu]

Thanks, my annoying 30% default backlight level will soon be history.

[ivy]

looks fine from the pkgbase side.

[ziaee]

Can you put this in rc.conf.5 too?