check the result of killing a container

the kill(2) may have some failure condition:

```
RETURN VALUE
       On success (at least one signal was sent), zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS
       EINVAL An invalid signal was specified.

       EPERM  The process does not have permission to send the signal to any of the target processes.

       ESRCH  The process or process group does not exist.  Note that an existing process might be a zombie, a process that has terminated execution, but has not yet been wait(2)ed for.
```

Signed-off-by: Wang Xu <gnawux@gmail.com>

src/init.c

@@ -640,10 +640,12 @@ static int hyper_new_container(struct hyper_pod *pod, char *json, int length)
 	return ret;
 }
 
-static int hyper_kill_container(struct hyper_pod *pod, char *json, int length)
+static int hyper_kill_container(struct hyper_pod *pod, char *json, int length, uint8_t **rmsg)
 {
 	struct hyper_container *c;
 	int ret = -1;
+	size_t message_len = 0;
+	const char *emsg = NULL;
 
 	JSON_Value *value = hyper_json_parse(json, length);
 	if (value == NULL) {
@@ -654,13 +656,36 @@ static int hyper_kill_container(struct hyper_pod *pod, char *json, int length)
 	c = hyper_find_container(pod, id);
 	if (c == NULL) {
 		fprintf(stderr, "can not find container whose id is %s\n", id);
+		emsg = "no such container";
 		goto out;
 	}
 
-	kill(c->exec.pid, (int)json_object_get_number(json_object(value), "signal"));
-	ret = 0;
+	ret = kill(c->exec.pid, (int)json_object_get_number(json_object(value), "signal"));
+	if (ret <0) {
+		switch(errno) {
+			case EINVAL:
+				emsg = "invalid signal";
+				break;
+			case EPERM:
+				emsg = "no permission";
+				break;
+			case ESRCH:
+				emsg = "no such process";
+				break;
+			default:
+				emsg = "kill failed";
+				break;
+		}
+	}
 out:
 	json_value_free(value);
+	if (emsg != NULL) {
+		message_len = strlen(emsg) + 1;
+		*rmsg = (uint8_t*)malloc(message_len);
+		if (*rmsg != NULL) {
+			strncpy((char*)*rmsg, emsg, message_len);
+		}
+	}
 	return ret;
 }
 
@@ -1209,7 +1234,10 @@ static int hyper_ctlmsg_handle(struct hyper_event *he, uint32_t len)
 		ret = hyper_new_container(pod, (char *)buf->data + 8, len - 8);
 		break;
 	case KILLCONTAINER:
-		ret = hyper_kill_container(pod, (char *)buf->data + 8, len - 8);
+		ret = hyper_kill_container(pod, (char *)buf->data + 8, len - 8, &data);
+		if (data != NULL) {
+			datalen = strlen((char*)data) + 1;
+		}
 		break;
 	case REMOVECONTAINER:
 		ret = hyper_remove_container(pod, (char *)buf->data + 8, len - 8);