varlink: add debug logging

When something fails, we need some logs to figure out what happened.
This is primarily relevant for connection errors, but in general we
want to log about all errors, even if they are relatively unlikely.

We want one log on failure, and generally no logs on success.
The general idea is to not log in static functions, and to log in the
non-static functions. Non-static functions which call other functions
may thus log or not log as appropriate to have just one log entry in the
end.