libsecureboot: allow OpenPGP support to be dormant

Since we can now add OpenPGP trust anchors at runtime,
ensure the latent support is available.

Ensure we do not add duplicate keys to trust store.

Also allow reporting names of trust anchors added/revoked

We only do this for loader and only after initializing trust store.
Thus only changes to initial trust store will be logged.

Reviewed by: stevek
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D20700