[HF] Fee output logic cleanup, allow 0-value fees

[instagibbs]

Previous logic, aside from allowing 0-fee outputs, was completely duplicated in other sections of the code.

The value-checking of outputs is already done in CheckTransaction, VerifyAmounts and VerifyCoinbaseAmount, so no need to check again in another context.

I can not recall why we disallowed 0-value fees initially, but the new logic is cleaner since we now always allow 0-value outputs as long as they're IsUnspendable, which fees are a smaller subset of.

I have also added more tests.