varlink: honour "sensitive" flag of json variant objects all the way into the socket

varlink: honour "sensitive" flag of json variant objects all the way into the socket

Let's honour the flag if it is set, just to be safe.

(This only handles the case for the writing side: whenever the client
code hands us a json object with the flag set we'll honour it till the
it's out of reach for us. This does *not* handle the reading side, which
is left for a later patch once needed. We probably should add a
per-connection flag that simply globally enables the sensitive logic for
all messages coming in on a specific varlink conneciton.)