extend_info_describe should list IPv6 address (if present)
When running an IPv6-only client using:
src/or/tor ClientUseIPv4 0 DataDirectory `mktemp -d` UseMicrodescriptors 0
I get log messages like
[info] add_an_entry_guard(): Chose $906933A309F15D7CF379127078A6791DF9B0C763~Unnamed at 126.96.36.199 as new entry guard
But I'm really contacting them on their IPv6 address. So we should list it along with the IPv4 address.