Kill exec process with SIGTERM instead of SIGKILL

In `runc.Exec`, [here](https://github.com/containerd/go-runc/blob/master/runc.go#L201) the exec command is created with `exec.CommandContext`, which will kill the `runc` command with `os.Process.Kill` when the context is done. And when `runc` is killed by `SIGKILL`, the exec process inside the container is not stopped. I think we should send `SIGTERM` manually to stop the `runc` command when we're doing exec.

I only tested this with `docker-runc`. Can anyone confirm whether it is the same with `runc`?