We should unity `-localhost` and `-ipv6host` into the same command line arguments. Looking to other "professional" software which also able to specify the IP in which listen to, they don't have separated command line arguments for that (e.g. the `--bind_ip` argument for mongod described in http://docs.mongodb.org/v2.2/reference/mongod/). If the difference between IPv4 and IPv6 matters, it should be our code (but not the user) the one dealing with it.