ntp: add pre-NTPv5 experimental extension field

Add an experimental extension field for some features that were proposed
for NTPv5. Higher-resolution root delay and dispersion (using 28-bit
fraction) are added. A monotonic receive timestamp will allow a
frequency transfer between the server and client. The client will be
able to separate the server's time corrections from frequency
corrections by tracking the offset between the real-time and monotonic
receive timestamps.

The field has a type of 0xF323 from the new experimental range proposed
by the NTP working group. Include a magic 32-bit value in the field to
avoid interoperability issues if a different implementation choses the
same type for its own experimental field. The value will be changed on
incompatible changes to avoid issues between two different chrony
versions.

ntp_core.c

@@ -1406,6 +1406,11 @@ parse_packet(NTP_Packet *packet, int length, NTP_PacketInfo *info)
       case NTP_EF_NTS_AUTH_AND_EEF:
         info->auth.mode = NTP_AUTH_NTS;
         break;
+      case NTP_EF_EXP1:
+        if (ef_body_length == sizeof (NTP_ExtFieldExp1) &&
+            ntohl(((NTP_ExtFieldExp1 *)ef_body)->magic) == NTP_EF_EXP1_MAGIC)
+          info->ext_field_flags |= NTP_EF_FLAG_EXP1;
+        break;
       default:
         DEBUG_LOG("Unknown extension field type=%x", (unsigned int)ef_type);
     }