libsecureboot: be more verbose about validation failures

Reviewed by: imp, sjg
Pull Request: https://github.com/freebsd/freebsd-src/pull/916

Remove $FreeBSD$: one-line sh pattern

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

bearssl: Suppress warnings from functions that have no args

clag15 is much pickier than others, so suppress the warning until
upstream can be updated.

Sponsored by: Netflix