ps: Support muitiple ps arguments

Before patch:
 | # ./runc ps test -- -e -f
 |   PID TTY          TIME CMD
 | 29046 pts/2    00:00:00 sh  <-- The -f option was skipped
 | #

After patch:
 | # ./runc ps test -- -e -f
 | UID        PID  PPID  C STIME TTY          TIME CMD
 | root     29046 29038  0 Jul06 pts/2    00:00:00 sh
 | #

Reported-by: Qiang Huang <h.huangqiang@huawei.com>

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>

ps.go

@@ -42,15 +42,21 @@ var psCommand = cli.Command{
 			return nil
 		}
 
-		psArgs := context.Args().Get(1)
-		if psArgs == "--" {
-			psArgs = context.Args().Get(2)
-		}
-		if psArgs == "" {
-			psArgs = "-ef"
+		// [1:] is to remove command name, ex:
+		// context.Args(): [containet_id ps_arg1 ps_arg2 ...]
+		// psArgs:         [ps_arg1 ps_arg2 ...]
+		//
+		psArgs := context.Args()[1:]
+
+		if len(psArgs) > 0 && psArgs[0] == "--" {
+			psArgs = psArgs[1:]
 		}
 
-		output, err := exec.Command("ps", strings.Split(psArgs, " ")...).Output()
+		if len(psArgs) == 0 {
+			psArgs = []string{"-ef"}
+		}
+
+		output, err := exec.Command("ps", psArgs...).Output()
 		if err != nil {
 			return err
 		}