Add basic fifo support for IO copy

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

io.go

@@ -0,0 +1,52 @@
+package containerd
+
+import (
+	"io"
+	"os"
+)
+
+type ioConfig struct {
+	StdoutPath string
+	StderrPath string
+	Stdin      io.WriteCloser
+	Stdout     io.ReadCloser
+	Stderr     io.ReadCloser
+}
+
+func newCopier(i *ioConfig) (*copier, error) {
+	l := &copier{
+		config: i,
+	}
+	if i.StdoutPath != "" {
+		f, err := os.OpenFile(i.StdoutPath, os.O_RDWR, 0)
+		if err != nil {
+			return nil, err
+		}
+		go io.Copy(f, i.Stdout)
+	}
+	if i.StderrPath != "" {
+		f, err := os.OpenFile(i.StderrPath, os.O_RDWR, 0)
+		if err != nil {
+			return nil, err
+		}
+		go io.Copy(f, i.Stderr)
+	}
+	return l, nil
+}
+
+type copier struct {
+	config *ioConfig
+}
+
+func (l *copier) Close() (err error) {
+	for _, c := range []io.Closer{
+		l.config.Stdin,
+		l.config.Stdout,
+		l.config.Stderr,
+	} {
+		if cerr := c.Close(); err == nil {
+			err = cerr
+		}
+	}
+	return err
+}