Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

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