varlink,json: introduce new varlink_dispatch() helper

varlink_dispatch() is a simple wrapper around json_dispatch() that
returns clean, standards-compliant InvalidParameter error back to
clients, if the specified JSON cannot be parsed properly.

For this json_dispatch() is extended to return the offending field's
name. Because it already has quite a few parameters, I then renamed
json_dispatch() to json_dispatch_full() and made json_dispatch() a
wrapper around it that passes the new argument as NULL. While doing so I
figured we should also get rid of the bad= argument in the short
wrapper, since it's only used in the OCI code.

To simplify the OCI code this adds a second wrapper oci_dispatch()
around json_dispatch_full(), that fills in bad= the way we want.

Net result: instead of one json_dispatch() call there are now:

1. json_dispatch_full() for the fully feature mother of all dispathers.
2. json_dispatch() for the simpler version that you want to use most of
   the time.
3. varlink_dispatch() that generates nice Varlink errors
4. oci_dispatch() that does the OCI specific error handling

And that's all there is.

src/fuzz/fuzz-bootspec.c

@@ -96,7 +96,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         if (r < 0)
                 return 0;
 
-        r = json_dispatch(v, data_dispatch, NULL, 0, &config);
+        r = json_dispatch(v, data_dispatch, 0, &config);
         if (r < 0)
                 return 0;