Make getauxblock return True on block submission to comply with pool software

`getauxblock` RPC call is modelled after `getwork` rather than `getblocktemplate`/`submitblock`, so pool software expects it to return `True` or an error string.

p2pool prints out an error message when receiving `None` instead of `True`, its other logic is unaffected.

powerpool does not update its internal statistics when receiving `None` instead of `True`, in this case it is an important issue.