ntp: add structure with packet info

Add a structure for length and other information about received and
transmitted NTP packets to minimize the number of parameters and avoid
repeated parsing of the packet.

ntp.h

@@ -110,6 +110,13 @@ typedef struct {
 #define NTP_REFID_LOCAL 0x7F7F0101UL /* 127.127.1.1 */
 #define NTP_REFID_SMOOTH 0x7F7F01FFUL /* 127.127.1.255 */
 
+/* Structure describing an NTP packet */
+typedef struct {
+  int length;
+  int version;
+  NTP_Mode mode;
+} NTP_PacketInfo;
+
 /* Structure used to save NTP measurements.  time is the local time at which
    the sample is to be considered to have been made and offset is the offset at
    the time (positive indicates that the local clock is slow relative to the