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/nspawn/nspawn-oci.c

@@ -2214,7 +2214,7 @@ int oci_load(FILE *f, const char *bundle, Settings **ret) {
 
         path = strjoina(bundle, "/config.json");
 
-        r = json_parse_file(f, path, &oci, &line, &column);
+        r = json_parse_file(f, path, 0, &oci, &line, &column);
         if (r < 0) {
                 if (line != 0 && column != 0)
                         return log_error_errno(r, "Failed to parse '%s' at %u:%u: %m", path, line, column);