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."

PR: 211425
Submitted by: clive