loader: allow for exceptions to restricted settings.

We restrict what an unverified loader.conf etc can set,
and the same restrictions are applied to interactive input.
We need to allow for exceptions (eg boot_verbose).
It is best if any allowed settings match up to '='.

If we do not allow it to be set, do not allow it to be unset

Reviewed by: stevek
Sponsored by: Juniper Networks, Inc.