Support I (idle) process state on procfs+Linux

`I` is a Linux process state: https://github.com/torvalds/linux/blob/master/fs/proc/array.c#L135

This is a fairly recent change but causes lots of Telegraf log noise with 4.x kernels.

plugins/inputs/system/processes.go

@@ -70,17 +70,15 @@ func getEmptyFields() map[string]interface{} {
 		"stopped":  int64(0),
 		"running":  int64(0),
 		"sleeping": int64(0),
+		"idle":     int64(0),
 		"total":    int64(0),
 		"unknown":  int64(0),
 	}
 	switch runtime.GOOS {
 	case "freebsd":
-		fields["idle"] = int64(0)
 		fields["wait"] = int64(0)
 	case "darwin":
-		fields["idle"] = int64(0)
 	case "openbsd":
-		fields["idle"] = int64(0)
 	case "linux":
 		fields["dead"] = int64(0)
 		fields["paging"] = int64(0)
@@ -174,6 +172,8 @@ func (p *Processes) gatherFromProc(fields map[string]interface{}) error {
 			fields["stopped"] = fields["stopped"].(int64) + int64(1)
 		case 'W':
 			fields["paging"] = fields["paging"].(int64) + int64(1)
+		case 'I':
+			fields["idle"] = fields["idle"].(int64) + int64(1)
 		default:
 			log.Printf("I! processes: Unknown state [ %s ] in file %s",
 				string(stats[0][0]), filename)