json: add flags parameter to json_parse_file(), for parsing "sensitive" data

This will call json_variant_sensitive() internally while parsing for
each allocated sub-variant. This is better than calling it a posteriori
at the end, because partially parsed variants will always be properly
erased from memory this way.

src/fuzz/fuzz-json.c

@@ -18,7 +18,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         f = fmemopen_unlocked((char*) data, size, "re");
         assert_se(f);
 
-        if (json_parse_file(f, NULL, &v, NULL, NULL) < 0)
+        if (json_parse_file(f, NULL, 0, &v, NULL, NULL) < 0)
                 return 0;
 
         g = open_memstream_unlocked(&out, &out_size);