fundrawtransaction fails when transaction already includes any number of inputs

[jrick]

This method should still be usable even when the transaction already contains inputs, by adding more inputs (as needed) and adding a change output for the appropriate fee.

Since part of the reason we want this to work is for a multisig workflow through the json-rpc interface, we also want size estimation working properly when spending p2sh outputs where we know the redeem script and how large the redemption will be (as is the case for multisig). This size estimation is a prerequisite for determining the correct transaction fee and change output value.