binding a socket for 18.0.0.1 to learn our interface address is freaking out users

People who use little snitch on OS X are being told that their Tor mysteriously sends udp packets to 18.0.0.1 on startup.

This is false -- no packets are sent. grep for 18.0.0.1 in src/common/address.c to learn what's going on.

But the number of reports from users is increasing. I imagine they think it looks like spyware behavior -- sekrit undocumented spyware behavior, too.

We should explore other ways to learn the user's interface address that don't trigger software like little snitch.