btcjson: Add spendable to listunspent result.
btcjson: Add spendable to listunspent result.

jcmincke/dcrd,jrick/dcrd,decred/dcrd,jrick/btcd,jrick/dcrd,jcmincke/dcrd,jrick/btcd,jrick/btcd,jcmincke/dcrd,decred/dcrd