send signal to exec process' ppid

because `nsenter-exec` forked the process that runs the user command, we dont
have a way to get the user process' pid. However, the user process will have
ppid = `nsenter-exec` pid which we know. This patch will send signal to ppid
instead so the user process can receive it.

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)