Use tor_addr_is_v4() rather than family, or reject all v6-mapped IPv4 addresses

In Tor, we try to support v6-mapped IPv4 addresses. We should either:

• reject them unconditionally, or
• audit all uses of tor_addr_t.family to see if we should be calling tor_addr_is_v4() instead, and add a comment to the struct that says we should consider using tor_addr_is_v4() rather than comparing family.

If no relay in the consensus is currently using these addresses, then maybe we should just call them internal on authorities, relays, and clients, and remove all the code that tries to deal with them.

Discovered as part of legacy/trac#15518 (moved).