Refactor runtime code for code sharing

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

pkg/stdio/stdio.go

@@ -0,0 +1,30 @@
+/*
+   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 stdio
+
+// Stdio of a process
+type Stdio struct {
+	Stdin    string
+	Stdout   string
+	Stderr   string
+	Terminal bool
+}
+
+// IsNull returns true if the stdio is not defined
+func (s Stdio) IsNull() bool {
+	return s.Stdin == "" && s.Stdout == "" && s.Stderr == ""
+}