Handle overvoting?

[danwallach]

Right now, if we're asked to encrypt an overvoted ballot, the resulting proofs will not correctly verify. Resolving this issue requires two things:

• interpreting an overvoted contest down to a blank input
• placing something meaningful into the extended data

The extended data is meant to hold write-in data, original (uninterpreted) votes, and other such things, all encrypted with hashed ElGamal but the spec doesn't provide detailed guidance on this.