varlink: automatically send ExpectedMore error message back when we were called witho...

varlink: automatically send ExpectedMore error message back when we were called without more=true set, but need it

Various Varlink calls only make sense if they are called with more=true
(i.e. in a mode where multiple replies are expected to be sent). If a
method call assumes it is called with more (manifested in the fact it
calls varlink_notify(), the call to reply to such messages) let's return
a recognizable error code for the violated expectation.

This adds a new error for this, org.varlink.service.ExpectedMore. Note
we are squatting the official org.varlink.service namespace, but for
such a basic thing it makes sense to add it there.