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_unix.go

@@ -0,0 +1,32 @@
+// +build !windows
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package v2
+
+import (
+	"context"
+	"io"
+	"path/filepath"
+
+	"github.com/containerd/fifo"
+	"golang.org/x/sys/unix"
+)
+
+func openShimLog(ctx context.Context, bundle *Bundle) (io.ReadCloser, error) {
+	return fifo.OpenFifo(ctx, filepath.Join(bundle.Path, "log"), unix.O_RDONLY|unix.O_CREAT|unix.O_NONBLOCK, 0700)
+}