varlink: optionally, mark all incoming message's "parameters" field as sensitive

varlink: optionally, mark all incoming message's "parameters" field as sensitive

So far the varlink logic honoured the "sensitive" flag of output
messages. Let's add something similar for input messages. Since we don't
really know incoming messages, the flag simply controls whether the
"parmaeters" field of all incoming messages should be marked as
sensitive.

Then, turn this on in the credentials logic and in homed, since both
deal with credentials.