added 'connected' cell

[Roger Dingledine <arma@mit.edu>]

Currently, an application proxy connects, sends addr/port, and if the
router can parse addr/port, it sends back a "0", meaning "everything's
ok, go ahead and start sending data now."

I've added a new cell type, 'connected'. Now the exit node sends back a
'connected' cell along the circuit, when the connection to the remote host
succeeds. It passes back, and when the entry node gets it, it writes a
"0" to the proxy as before.

The benefit here is that the client has a bit more hope of diagnosing if
"it doesn't work" means "the remote host was down" or "the onion didn't
work". In theory the client should notice when it's been too long since
he started the connection, and do something about that. Soon.

--Roger