Change default RPC port to 8334 (18334 testnet).

This change paves the way for running btcwallet on the same system without
having to change any settings.  The well-known ports used by the
reference implementation (8332 mainnet, 18332 testnet) will be exposed by
the separate wallet process, which will in turn forward unknown requests
to btcd via websockets (on 8334/18334).  This allows the wallet process to
ultimately provide a unified interface that exposes the same RPC-JSON API
as the reference implementation will maintaining wallet and chain
separation.