ntp: prepare for non-authentication extension fields

Add a new variable to the packet info structure with flags for extension
fields included in received packets and add a new parameter to
transmit_packet() to add the fields to transmitted packets.

ntp.h

@@ -137,6 +137,7 @@ typedef struct {
   NTP_Mode mode;
 
   int ext_fields;
+  int ext_field_flags;
 
   struct {
     NTP_AuthMode mode;