libct: replace unix.Kill with os.Process.Signal

Because we should switch to unix.PidFDSendSignal in new kernels, it has
been supported in go runtime. We don't need to add fall back to
unix.Kill code here.

Signed-off-by: lifubang <lifubang@acmcoder.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/process_linux.go

@@ -116,11 +116,7 @@ func (p *containerProcess) startTime() (uint64, error) {
 }
 
 func (p *containerProcess) signal(sig os.Signal) error {
-	s, ok := sig.(unix.Signal)
-	if !ok {
-		return errors.New("os: unsupported signal type")
-	}
-	return unix.Kill(p.pid(), s)
+	return p.cmd.Process.Signal(sig)
 }
 
 func (p *containerProcess) externalDescriptors() []string {