In this change, we return 404 if the request path is neither /json-rpc
nor /rpc. If XML feature is not enabled and /rpc is requested, return
404. If XML parser failed, return 400. JSON parser failure has been
handled well in the existing code.
Formally, depending on the method implementation, this kind of error
is just ignored and parameter is skipped. Now aria2 responds error for
these cases. The required parameter checking is also reworked along
with this change.