Add shim log pipe for log forwarding to the daemon

A fifo on unix or named pipe on Windows will be provided to the shim.
It can be located inside the `cwd` of the shim named "log".
The shims can use the existing `github.com/containerd/containerd/log` package to log debug messages.
Messages will automatically be output in the containerd's daemon logs with the correct fiels and runtime set.

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

runtime/v2/shim/shim_unix.go

@@ -21,6 +21,7 @@ package shim
 import (
 	"bytes"
 	"context"
+	"io"
 	"net"
 	"os"
 	"os/exec"
@@ -29,6 +30,7 @@ import (
 
 	"github.com/containerd/containerd/events"
 	"github.com/containerd/containerd/namespaces"
+	"github.com/containerd/fifo"
 	"github.com/containerd/typeurl"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
@@ -84,6 +86,10 @@ func handleSignals(logger *logrus.Entry, signals chan os.Signal) error {
 	}
 }
 
+func openLog(ctx context.Context, _ string) (io.Writer, error) {
+	return fifo.OpenFifo(context.Background(), "log", unix.O_WRONLY, 0700)
+}
+
 func (l *remoteEventsPublisher) Publish(ctx context.Context, topic string, event events.Event) error {
 	ns, _ := namespaces.Namespace(ctx)
 	encoded, err := typeurl.MarshalAny(event)