varlink: add two helpers for delayed processing of method calls

varlink: add two helpers for delayed processing of method calls

When we want to do Polkit authentication we want to temporarily pause
handling of a method call until we have the Polkit reply, and then start
again. Let's add some glue to make that easy. This adds two helpers:

varlink_dispatch_again() allows to ask for redispatching of the
currently queued incoming message. Usecase is this: if we don't process
a methd right away, we can come back later, and ask it to be processed
again with this function, in which case our handlers will be called a
2nd time, exactly like on the first time.

varlink_get_current_message() provides access to the currently processed
method call.

With this the polkit logic can look into the current message, do its
thing, and then restart the method handling.