Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

backlight(8): Update usage() to match the manual page

MFC after: 1 week

backlight: Remove set but not used variable

backlight: Fix incr/decr with percent value of 0

This now does nothing instead of incr/decr by 10%

MFC After: 3 days
PR: 253736

backlight: check the lenght if the input before trimming '%'

Reported by: hps
Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D26693

backlight: accept '%' in the brightness input value

Improve friendlyness of the command line by accepting the percent brightness
in both format: with or without a trailing '%'

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D26692

Add backlight(8)

This tool is used to configure registered backlights.
It can incr/decr (default to 10%) or accept a percentage value directly.

Reviewed by: manpages (gbe@)
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26251