json: replace JSON_FORMAT_REFUSE_SENSITIVE with JSON_FORMAT_CENSOR_SENSITIVE

json: replace JSON_FORMAT_REFUSE_SENSITIVE with JSON_FORMAT_CENSOR_SENSITIVE

Previously, the flag would completely refuse formatting a JSON object if
any field of it was marked sensitive. With this change we'll simply
replace the subobject with the string "<sensitive data>", and show
everything else.

This is tremendously useful when debugging, since it means that we can
again trace varlink calls through the stack: we can show all the message
metadata and just suppress the actually sensitive parameters.

The ability to debug this matters, and we should not hide more
information that we can get away with, to keep things debuggable and
maintainable.