Add missing monitor file

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

vendor/github.com/crosbymichael/go-runc/monitor.go

@@ -0,0 +1,50 @@
+package runc
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+var Monitor ProcessMonitor = &defaultMonitor{}
+
+// ProcessMonitor is an interface for process monitoring
+//
+// It allows daemons using go-runc to have a SIGCHLD handler
+// to handle exits without introducing races between the handler
+// and go's exec.Cmd
+// These methods should match the methods exposed by exec.Cmd to provide
+// a consistent experience for the caller
+type ProcessMonitor interface {
+	Output(*exec.Cmd) ([]byte, error)
+	CombinedOutput(*exec.Cmd) ([]byte, error)
+	Run(*exec.Cmd) error
+	Start(*exec.Cmd) error
+	Wait(*exec.Cmd) (int, error)
+}
+
+type defaultMonitor struct {
+}
+
+func (m *defaultMonitor) Output(c *exec.Cmd) ([]byte, error) {
+	return c.Output()
+}
+
+func (m *defaultMonitor) CombinedOutput(c *exec.Cmd) ([]byte, error) {
+	return c.CombinedOutput()
+}
+
+func (m *defaultMonitor) Run(c *exec.Cmd) error {
+	return c.Run()
+}
+
+func (m *defaultMonitor) Start(c *exec.Cmd) error {
+	return c.Start()
+}
+
+func (m *defaultMonitor) Wait(c *exec.Cmd) (int, error) {
+	status, err := c.Process.Wait()
+	if err != nil {
+		return -1, err
+	}
+	return status.Sys().(syscall.WaitStatus).ExitStatus(), nil
+}