Mlat synthetic message detection.

Notice synthetic mlat messages by looking for messages with a magic
timestamp value. If they arrive, tag the derived data as mlat-derived.

Don't include mlat-derived output in FATSV output to avoid loops.

mode_s.c

@@ -59,6 +59,11 @@
 //
 //
 
+/* A timestamp that indicates the data is synthetic, created from a
+ * multilateration result
+ */
+#define MAGIC_MLAT_TIMESTAMP 0xFF01A71A71A7ULL
+
 //=========================================================================
 //
 // Given the Downlink Format (DF) of the message, return the message length in bits.
@@ -563,6 +568,9 @@ int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
 
     mm->bFlags = 0;
 
+    if (mm->remote && mm->timestampMsg == MAGIC_MLAT_TIMESTAMP)
+        mm->bFlags |= MODES_ACFLAGS_FROM_MLAT;
+
     // AA (Address announced)
     if (mm->msgtype == 11 || mm->msgtype == 17 || mm->msgtype == 18) {
         mm->addr  = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);