Allows the user to pass null as the second or third parameter
to signrawtransaction, in case you need to (for example) fetch
private keys from the wallet but want to specify the hash type.
Signrawtransaction rpc was crashing when some inputs were unknown,
and even with that fixed was failing to handle all the known inputs
if there were unknown inputs in front of them. This commit instead
attempts to fetch inputs one at a time.
Implement listunspent / getrawtransaction / createrawtransaction /
signrawtransaction, to support creation and
signing-on-multiple-device multisignature transactions.
