Move environment configuration to Process

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

process.go

@@ -1,6 +1,9 @@
 package libcontainer
 
-import "io"
+import (
+	"io"
+	"os/exec"
+)
 
 // Process specifies the configuration and IO for a process inside
 // a container.
@@ -8,6 +11,9 @@ type Process struct {
 	// The command to be run followed by any arguments.
 	Args []string
 
+	// Env specifies the environment variables for the process.
+	Env []string
+
 	// Stdin is a pointer to a reader which provides the standard input stream.
 	Stdin io.Reader
 
@@ -16,4 +22,6 @@ type Process struct {
 
 	// Stderr is a pointer to a writer which receives the standard error stream.
 	Stderr io.Writer
+
+	cmd *exec.Cmd
 }