Add a relay-v6.tmpl template file
For some reasons,
exit-v6.i which now requires
ipv6_addr to be set else
None is put in place.
basic-min fails because it ends up with
ORPort None:5001 IPv6Only. Instead, make
relay.tmpl to be IPv4 only and
relay-v6.tmpl to be v4 and v6.
This makes all network configuration using
relay.tmpl to work normally with IPv4 only.