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.
