Compressed packets have significant attack surface, and are forbidden
in certificates, keys, and detached signatures. When parsing detached
signatures, forbid any packet that is not a signature or marker packet.
When parsing certificates, return an error when encountering a
compressed packet, instead of decompressing the packet.
Furthermore, certificates, keys, and signatures are not allowed to
contain partial-length or indeterminate-length packets. Reject those in
parse_packet, rather than activating the partial-length filter code.