ntp: add client support for network correction

If the network correction is known for both the request and response,
and their sum is not larger that the measured peer delay, allowing the
transparent clocks to be running up to 100 ppm faster than the client's
clock, apply the corrections to the NTP offset and peer delay. Don't
correct the root delay to not change the estimated maximum error.

candm.h

@@ -278,6 +278,7 @@ typedef struct {
 #define REQ_ADDSRC_NTS 0x200
 #define REQ_ADDSRC_COPY 0x400
 #define REQ_ADDSRC_EF_EXP_MONO_ROOT 0x800
+#define REQ_ADDSRC_EF_EXP_NET_CORRECTION 0x1000
 
 typedef struct {
   uint32_t type;