Enable --mainnet flag.

This change enables the --mainnet flag to connect to a mainnet btcd
instance and open and create mainnet wallets.  Attempting to connect
to a testnet btcd, or opening a testnet wallet when running in mainnet
mode will result in an error printed to the logging output.

Testnet (version 3) remains the default network for now.  btcwallet
will continue to receive fixes and new features, but at the moment we
believe that early adopters familar with using btcwallet on testnet
can safely use it for mainnet now as well.

Reminder: when dealing with mainnet coins, it is always a good idea to
keep backups of your wallet.  btcwallet uses a deterministic wallet,
and any later addresses for an account can be recreated with just the
account's original wallet file (this does *not* include imported
addresses).  Might we recommend Cyphertite for a backup solution?