Relay OP_RETURN TxOut as standard transaction type

Also fix decoderawtransaction to not show reqSigs or addresses for
nulldata txouts. (Previous version also left reqSigs uninitialized
mistakenly)
Show short scriptPubKeys correctly

Previously bitcoin-qt's -debug transaction info was showing CTxOut([error])

It is valid for a scriptPubKey to be any size, for example simply
OP_RETURN is valid and can be used to destroy a TXOUT to mining fees.
Merge pull request #3128

0056095 Show short scriptPubKeys correctly (Peter Todd)
22de68d Relay OP_RETURN TxOut as standard transaction type (Peter Todd)

Signed-off-by: Gavin Andresen <gavinandresen@gmail.com>