relay: Automatically Enable an IPv6 ORPort

This commit makes it that if the ORPort is set with a single port, it will
bind to both global listen IPv4 and IPv6 addresses.

To pin an "ORPort <PORT>" to be IPv4 or IPv6, the IPv4Only/IPv6Only flags are
honored thus this will _only_ bind on IPv6 for that port value:

  ORPort 9050 IPv6Only
    Results in: [::]:9050

  ORPort 9051 IPv4Only
    Results in: [0.0.0.0]:9051

Attempting to configure an explicit IPv4 address with IPv6Only flag is an
error and vice versa.

Closes #33246

Signed-off-by: David Goulet <dgoulet@torproject.org>