Portability fixes for containerd shim

Update go-runc to master with portability fixes.

Subreaper only exists on Linux, and only Linux runs the shim in a
mount namespace.

With these changes the shim compiles on Darwin, which means the
whole build compiles without errors now.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

vendor/github.com/containerd/go-runc/command_linux.go

@@ -0,0 +1,21 @@
+package runc
+
+import (
+	"context"
+	"os/exec"
+	"syscall"
+)
+
+func (r *Runc) command(context context.Context, args ...string) *exec.Cmd {
+	command := r.Command
+	if command == "" {
+		command = DefaultCommand
+	}
+	cmd := exec.CommandContext(context, command, append(r.args(), args...)...)
+	if r.PdeathSignal != 0 {
+		cmd.SysProcAttr = &syscall.SysProcAttr{
+			Pdeathsig: r.PdeathSignal,
+		}
+	}
+	return cmd
+}