load_unknown_x509_certificate() serves for the cases where we
can't be sure what the format of its input certificate is. This
is the case for installers, it should not be used anywhere else.
Reviewed-By: Fraser Tweedale <email@example.com>
Reviewed-By: Rob Crittenden <firstname.lastname@example.org>
Reviewed-By: Martin Basti <email@example.com>