onion: endpoint fallbacks + ephemeral

[meejah]

The "onion:" server-side listeners currently do the following:

1. if you specified a control-port in the onion-string, connect to that tor
2. call get_global_tor() and use the launched tor

As ephemeral services become available, it is possible to connect to system (or TBB) Tors to add onion services. This means: pass the private-key material in the string and:

1. if get_global_tor() ever got called, use that tor
2. try 9051 as a control-port, and add the onion if that worked
3. try 9151 as a control-port and add the onion if that worked
4. call get_global_tor ourselves and launch a new tor

[david415]

sounds good!