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: lfbzhm <[email protected]>

Author: lifubang

libcontainer/process_linux.go

@@ -115,11 +115,7 @@ func (p *setnsProcess) startTime() (uint64, error) {
 }
 
 func (p *setnsProcess) 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 *setnsProcess) start() (retErr error) {
@@ -838,11 +834,7 @@ func (p *initProcess) createNetworkInterfaces() error {
 }
 
 func (p *initProcess) 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 *initProcess) setExternalDescriptors(newFds []string) {