Tweak TSK description to allow for missing secret key material.

Sometimes, a TSK has to be transferred even if not all secrets are known or should be transferred. Notable examples are "offline primary keys", subkeys with secrets on a smart card, or additional archival encryption keys.

The current practice is to use a secret-key packet, but include a stub secret key. This seems unfortunate, because a) it has not been standardized, and b) this seems to be a modeling error: it is a secret-key packet with only public-key material, and should thus be a public-key packet.

Fixes #52 (closed).