varlink: allow using varlink_wait() even with a server

varlink: allow using varlink_wait() even with a server

This call can be useful even if a server object is declared.

(Originally this was not supported, because a server typically needs to
handle multiple connections, and thus a synchronous wait on one would
starve the others out. But in some cases it might make sense to have
varlink point-to-point connections — i.e. where the server only handles
a single connection ever — and there it makes sense to synchronously
wait on the one connection).