NTP: Sync time correctly when receiving broadcast updates

1) The value of rtems_bsdnet_ntpserver_count is equal to 0 when no
   server is set, so the check for (rtems_bsdnet_ntpserver_count < 0)
   in rtems_bsdnet_get_ntp() is wrong.  The check should be "<= 0".

2) Binding the listening socket port to 0 does not work.  Packets
   appear on the interface, but the recvfrom in tryServer() never
   returns.  Changing this to the well known NTP socket 123 allows
   the packets to be seen.

3) In tryServer(), an explicit check for NTP version 3 packets is made.
   If the NTP server is version 4, this check fails even though the
   packets seem to be the right shape.