Remove $FreeBSD$: one-line sh pattern

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

Make the uefikeys script output slightly more obvious.

MFC after: 1 month
Sponsored by: The FreeBSD Foundation

Add key/cert generation script for uefisign(8).

(Forgot about Relnotes in the commit that added uefisign(8), so set it here.)

MFC after: 1 month
Relnotes: yes
Sponsored by: The FreeBSD Foundation