Make Tor clients tell dual-stack exits that they prefer IPv6
connections.
This change is equivalent to setting the PreferIPv6 flag on
SOCKSPorts (and most other listener ports). Tor Browser has
been setting this flag for some time, and we want to remove a
client distinguisher at exits.
Also update the man page, putting all the IP flags in their
non-default forms.
Closes ticket 32637.
