Add null io option

This adds null IO option for efficient handling of IO.
It provides a container directly with `/dev/null` and does not require
any io.Copy within the shim whenever a user does not want the IO of the
container.

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

linux/shim/process.go

@@ -17,6 +17,10 @@ type stdio struct {
 	terminal bool
 }
 
+func (s stdio) isNull() bool {
+	return s.stdin == "" && s.stdout == "" && s.stderr == ""
+}
+
 type process interface {
 	// ID returns the id for the process
 	ID() string