Make utils fully UCSPI compliant

https://cr.yp.to/proto/ucspi.txt

Ideas:
- tgserver should be a UCSPI server
- tgsend/tgrecv could maybe be UCSPI clients? Not sure yet

Implications:
- All tools need a token in every invocation. Idea: list of aliases (name=token) in global config.
- tgserver will be able to keep sessions. Spawned per user? Per chat?