Implement a number of improvments:
- When shutdown, the correct behavior is to send SIGHUP.
- When PID of the gw6c process greater than 65536, it can not perform timeout ping check correctly, due to echo_id in icmp_hdr is declared as uint16_t.
- Mute dmesg noise "nd6_dad_timer: cancel DAD on tun0 because of ND6_IFF_IFDISABLED."
Submitted by: clive