change the user of the Process to a json.RawMessage

The type of `user` becomes `json.RawMessage` instead of platform aware
structure, so the Spec becomes a platform-independent structure.

It makes we can parse the config.json as a Spec before knowing
the os, and allows we move the linux related *.go to linux/.

The runtime should parse the user with the corresponding platform
specific User structure.

Signed-off-by: Lai Jiangshan <[email protected]>

Author: laijs

config.go

@@ -1,5 +1,9 @@
 package specs
 
+import (
+	"encoding/json"
+)
+
 // Spec is the base configuration for the container.  It specifies platform
 // independent configuration.
 type Spec struct {
@@ -21,8 +25,8 @@ type Spec struct {
 type Process struct {
 	// Terminal creates an interactive terminal for the container.
 	Terminal bool `json:"terminal"`
-	// User specifies user information for the process.
-	User User `json:"user"`
+	// User specifies user information for the process. It is a platform-specific structure.
+	User json.RawMessage `json:"user"`
 	// Args specifies the binary and arguments for the application to execute.
 	Args []string `json:"args"`
 	// Env populates the process environment for the process.