Add --throttle option.

This gives access to the existing throttling behaviour when reading
from a file with --interactive, without needing to actually have
--interactive.

This is useful when testing, as without --throttle samples will be
processsed much faster than real-time, which can produce different
results with e.g. aircraft/filter expiry times and position/speed checks.

dump1090.h

@@ -339,6 +339,7 @@ struct {                             // Internal state
     char *json_dir;                  // Path to json base directory, or NULL not to write json.
     uint64_t json_interval;          // Interval between rewriting the json aircraft file, in milliseconds; also the advertised map refresh interval
     int   json_location_accuracy;    // Accuracy of location metadata: 0=none, 1=approx, 2=exact
+    int   throttle;                  // When reading from a file, throttle file playback to realtime?
 
     int   json_aircraft_history_next;
     struct {