gpg: Don't take the a TOFU trust model from the trustdb,

* g10/tdbio.c (tdbio_update_version_record): Never store a TOFU model.
(create_version_record): Don't init as TOFU.
(tdbio_db_matches_options): Don't indicate a change in case TOFU is
stored in an old trustdb file.

This change allows to switch between a tofu and pgp or tofu+pgp trust
model without an auto rebuild of the trustdb. This also requires that
the tofu trust model is requested on the command line. If TOFU will
ever be the default we need to tweak the model detection via TM_AUTO
by also looking into the TOFU data base,

• GnuPG-bug-id: T4134