When ClientPreferIPv6ORPort is set to auto, and a relay is being chosen for a directory or orport connection, prefer IPv4 or IPv6 at random

Suggested by teor at legacy/trac#17835 (moved):

1. Make ClientPreferIPv6ORPort into an autobool
2. When ClientPreferIPv6ORPort is set to auto, and a relay is being chosen for a directory or orport connection, prefer IPv4 or IPv6 at random