procd: stop service using SIGKILL if SIGTERM failed to do so

SIGKILL is sent if instance process is still running after
<term_timeout> seconds after SIGTERM has been sent. To prevent
another daemon process being launched before old process dies,
the instance is kept until SIGCHLD confirms that service has
been stopped.

Signed-off-by: Alin Nastac <alin.nastac@gmail.com>