SNTP_CHECK_RESPONSE macro

Sanity check: Define this to - 0 to turn off sanity checks (default; smaller code) - >= 1 to check address and port of the response packet to ensure the response comes from the server we sent the request to. - >= 2 to check returned Originate Timestamp against Transmit Timestamp sent to the server (to ensure response to older request). - >= 3 @todo: discard reply if any of the VN, Stratum, or Transmit Timestamp fields is 0 or the Mode field is not 4 (unicast) or 5 (broadcast). - >= 4 @todo: to check that the Root Delay and Root Dispersion fields are each greater than or equal to 0 and less than infinity, where infinity is currently a cozy number like one second. This check avoids using a server whose synchronization source has expired for a very long time.