exit policy should allow 'self' address

In our microdescriptor work, one of the roadblocks to putting addresses (not just ports) in the exit policy is that some relays change their address quite often, and by default they have a "reject :*" line.

How about if we add a new keyword 'self' that is interpreted as the IP address claimed in the descriptor? Then the exit policy wouldn't need to change when the relay's address does.

I'm putting in 'tor unspecified' milestone because this rollout could take quite a while, and we should ponder if it would be moot by then.