fix signal handling on ping6 -f <nonexisting peer>.  from hash@iij.ad.jp
fix signal handling on ping6 -f <nonexisting peer>.  from hash@iij.ad.jp

MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame