Choose correct ports for testnet and mainnet.

This copies the functionality of btcd for choosing the ports for the
HTTP client (for btcd) and server (for frontends).  On testnet, the
following ports are used as default:

 - btcd: 18334
 - frontends: 18332

When running with the (currently disabled) --mainnet flag, btcwallet
will choose the following ports by default:

 - btcd: 8334
 - frontends: 8332

Both ports can be overridden no matter the chosen network using the -b
and -p flags.