Implement container signaling

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

linux_process.go

@@ -32,6 +32,8 @@ type parentProcess interface {
 
 	// startTime return's the process start time.
 	startTime() (string, error)
+
+	signal(os.Signal) error
 }
 
 type setnsProcess struct {
@@ -47,6 +49,10 @@ func (p *setnsProcess) startTime() (string, error) {
 	return system.GetProcessStartTime(p.pid())
 }
 
+func (p *setnsProcess) signal(s os.Signal) error {
+	return p.forkedProcess.Signal(s)
+}
+
 func (p *setnsProcess) start() (err error) {
 	defer p.parentPipe.Close()
 	if p.forkedProcess, err = p.execSetns(); err != nil {
@@ -260,3 +266,7 @@ func (p *initProcess) newUsernsSetupProcess() (parentProcess, error) {
 		config:     p.config,
 	}, nil
 }
+
+func (p *initProcess) signal(s os.Signal) error {
+	return p.cmd.Process.Signal(s)
+}