Listen on IPv6 by default for SocksPort *:Port

That would be very useful if tor could listen on both IPv4 and IPv6 for the SocksPort. One use case is for Torsocks to work seamlessly with v4 and v6 without having to configure a v6 port in the configuration file and restart the daemon.

One way to fix that would be to change the function parse_port_config() in src/or/config.c to allow multiple default values adding v6 defaults (which would benefit other ports as well).

Else, having a check somewhere that adds other defaults for specific ports but not sure that it's a good idea nor makes sense in the long run in terms of maintainability.

I thought also about bringing back legacy/trac#4760 (moved) by default and setting the ipv6 only option only and only if the user has defined an option in the torrc explicitly. For instance, this in the torrc would ONLY allow v6 (remove dual stack).

SocksPort [::1]:9050

Thoughts?