tweak the interface to generating signatures yet again.

tweak the interface to generating signatures yet again.
- assume key names match, deduce signer from sec key.
e.g., -s signify -s 55pkg.sec
will set signer to 55pkg and look for a pubkey named 55pkg.pub,
either besides 55pkg.sec or in /etc/signify.
- verify there's no mismatch, if possible, by verifying the first package
signed.

- also build a SHA256 on the fly while signing.