json-util: fine tune json_dispatch_log_level()

json-util: fine tune json_dispatch_log_level()

Let's give the user control on how to handle JSON "null" assignments of
the log level. As one of three cases: as failure, as LOG_NULL (i.e. to
turn off logging) or as LOG_INFO (as our usual default log level).

Let's then use that in the generic SetLogLevel() call, so that callers
can use it to explicitly turn off logging in a service.

Note that this was (probably accidentally) already implemented, except
that the introspection enforcement blocked it. Let's clean this up and
make this officially a thing, since it's generally useful to turn off
logging I think.