Perform Basic Relay IPv6 Extends
Currently, tor checks that extend cells have IPv4 addresses in: [x] some functions in circuitbuild_relay.c (a new file introduced by legacy/trac#33633 (moved)) [x] channel_get_for_extend() in channel.c [x] check_extend_cell() in onion.c [x] extend_cell_from_extend2_cell_body() in onion.c [?] and possibly other functions.
We want to allow IPv6 in all these places.
And when we have an IPv4 and an IPv6 address, we want to choose between them at random.
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