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>

cmd/containerd-shim/shim_unix.go

@@ -0,0 +1,27 @@
+// +build !linux
+
+package main
+
+import (
+	"os"
+	"os/signal"
+
+	"github.com/containerd/containerd/reaper"
+	runc "github.com/containerd/go-runc"
+)
+
+// setupSignals creates a new signal handler for all signals and sets the shim as a
+// sub-reaper so that the container processes are reparented
+func setupSignals() (chan os.Signal, error) {
+	signals := make(chan os.Signal, 2048)
+	signal.Notify(signals)
+	// make sure runc is setup to use the monitor
+	// for waiting on processes
+	runc.Monitor = reaper.Default
+	return signals, nil
+}
+
+// setupRoot is a no op except on Linux
+func setupRoot() error {
+	return nil
+}