Copy btcd RPC listening behavior.

This change copies the listening behavior of btcd by replacing the
--serverport option with --listen.  By default, btcwallet will only
listen for localhost connections, but with this change it will be
possible to add listeners for remote connections.

This was added due to finding a bug with updateConfigWithActiveParams.
After consulting the btcd source code, the bug was fixed by replacing
the function (as it was no longer needed) when the new listening code
was introduced.

While here, mask out the password flag from being shown in the help
message.