Bug 20261: Treat AF_UNIX addresses as equal when comparing them.

This is a kludge to deal with the fact that `tor_addr_t` doesn't contain
`sun_path`.  This currently ONLY happens when circuit isolation is being
checked, for an isolation mode that is force disabled anyway, so the
kludge is "ugly but adequate", but realistically, making `tor_addr_t`
and the AF_UNIX SocksPort code do the right thing is probably the better
option.