Successful multi-signature transaction validation (RIPD-480]

I've seen one multi-signed transaction succeed with this code.
And I've seen no failures (yet).  However there is stuff further
down stream from the signature check that causes the transaction
to fail.  So we're not out of the woods yet.

Submitting to github so reviewers can tell me if the code is in
the right place and what they would like to have changed.