Add dsa_check_key() calls on DSA decoding

Add dsa_check_key() calls on DSA decoding

When decoding a public or a private key, use dsa_check_key() to ensure
consistency of the DSA parameters. We do not always have sufficient
information to do that, so this is not always possible.

This adds new checks and replaces incomplete existing ones. On decoding
the private key we will now only calculate the corresponding public key,
if the sizes are sensible. This avoids potentially expensive operations.

ok beck jsing