Find teammates' hosts on other ports, such as stable and Canary #542

Hosts find teammates' hosts through tailscale status and connect to them on their own listener's port (apps/host/src/peers.ts:11). Stable listens on 4140 and Canary on 4142, so a stable host and a Canary host never see each other's channels directly. Only the directory's listings connect them, and they have no live peer connection.

A teammate on a different channel than you, or anyone running a dev build, is invisible until a directory exists. The same goes for any team that tries peer-only use without a directory.

Peers should be found whatever port they listen on. For example, a host could advertise its port, or probe the known ports.