A number of the struct definitions had broken tag definitions, which
meant some parsing was working by accident and some parsing was entirely
broken. Fixing this uncovered some additional issues (a mixture of
incorrect definitions and platform certificates that violate the spec),
so this is all cleaned up as well.
Attribute certificates are generally using RSA-SHA1 or RSA-SHA256
signatures, so include those. In addition, the CA signing restrictions
imposed for general purpose certificates don't apply here - drop that
restriction from certificate signature validation.
Platform certificates are defined as RFC5755 attribute certificates with
various additional attributes and extensions defined in the TCG Platform
Certificate Profile. Add support for parsing them, derived from
crypto/x509. Include some test certificates and verify we parse them.
