[tor-bugs] #17840 [Tor]: Add a minimal implementation of ClientUseIPv4 so IPv6-only clients can bootstrap

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#17840: Add a minimal implementation of ClientUseIPv4 so IPv6-only clients can
bootstrap
------------------------+--------------------------------
     Reporter:  teor    |      Owner:
         Type:  defect  |     Status:  new
     Priority:  Medium  |  Milestone:  Tor: 0.2.8.x-final
    Component:  Tor     |    Version:
     Severity:  Normal  |   Keywords:  ipv6
Actual Points:          |  Parent ID:  #17811
       Points:          |    Sponsor:
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 When Tor bootstraps, it connects to directory servers on their IPv4
 address, regardless of the ClientUseIPv6 option.

 We can add a ClientUseIPv4 option, and set it to 1 by default. (This needs
 a config check: reject the config if ClientUseIPv4 and ClientUseIPv6 are
 both 0.)

 Then we can add connection_get_address_family_for_purpose(), which returns
 the address family that should be used by outgoing connections. In this
 minimal implementation, we'll return AF_INET, unless ClientUseIPv4 is 0
 and ClientUseIPv6 is 1 (an IPv6-only client).

 For the moment, we'll ignore ClientPreferIPv6ORPort in
 connection_address_family_for_purpose. Once IPv6-only clients can
 bootstrap, we'll find out if the existing implementation of
 ClientPreferIPv6ORPort works.

 This will implement parts of #9068, but only the minimal functionality
 needed to decide whether we are allowed to connect via IPv4 or IPv6.
 (There are many clever things we could do on dual-stack clients in
 connection_address_family_for_purpose(), to guess which IP version to use,
 or to alternate IP versions to find the best one. Some of these designs
 are described in #9068 and #17834. But first we need to get IPv6-using and
 IPv6-only clients bootstrapping.)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17840>
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