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 #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
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