pcap logging; Timestamps wrapping after 59 seconds.

The datetime object does not hold total seconds since Epoch time.The seconds variable is only seconds since last full minute. We need
a deltatime since Epoch start to get total seconds since then

cflib/crtp/pcap.py

@@ -139,6 +139,7 @@ def _assemble_record(self, link_type, receive, address, channel, devid, crtp_pac
         )
 
     def _pcap_header(self, len):
-        ts = datetime.now()
-
-        return struct.pack('<LLLL', ts.second, ts.microsecond, len, len)
+        now = datetime.now()
+        delta = now - datetime(1900, 1, 1)
+        seconds = int(delta.total_seconds())
+        return struct.pack('<LLLL', seconds, delta.microseconds, len, len)