libct/system: add I and P process states

Those states are available since Linux 4.14 (kernel commits
8ef9925b02c23e3838d5 and 06eb61844d841d003). Before this
patch, they were shown as unknown.

This is mostly cosmetical.

Note that I is described in /proc/pid/status as just "idle", although
elsewhere it says it's an idle kernel thread. Let's have it as "idle"
for brevity.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/system/proc.go

@@ -19,6 +19,8 @@ const ( // Only values for Linux 3.14 and later are listed here
 	Stopped     State = 'T'
 	TracingStop State = 't'
 	Zombie      State = 'Z'
+	Parked      State = 'P'
+	Idle        State = 'I'
 )
 
 // String forms of the state from proc(5)'s documentation for
@@ -39,6 +41,10 @@ func (s State) String() string {
 		return "tracing stop"
 	case Zombie:
 		return "zombie"
+	case Parked:
+		return "parked"
+	case Idle:
+		return "idle" // kernel thread
 	default:
 		return fmt.Sprintf("unknown (%c)", s)
 	}