loader: ignore some variable settings if input unverified

libsecureboot can tell us if the most recent file opened was
verfied or not.
If it's state is VE_UNVERIFIED_OK, skip if variable
matches one of the restricted prefixes.

MFC of r350099

Reviewed by: stevek
Sponsored by: Juniper Networks
Differential Revision: https://reviews.freebsd.org//D20909