dhcp: ensure that dhcp client is exited

On restart ensure that the client we're trying to kill has
actually exited even if it's not our child.

src/dhcp-manager/nm-dhcp-client.c

@@ -156,12 +156,16 @@ stop_process (GPid pid, const char *iface)
 
 		if (ret == -1) {
 			/* Child already exited */
-			if (errno == ECHILD)
+			if (errno == ECHILD) {
+				/* Was it really our child and it exited? */
+				if (kill (pid, 0) < 0 && errno == ESRCH)
 					break;
+			} else {
 				/* Took too long; shoot it in the head */
 				i = 0;
 				break;
 			}
+		}
 		g_usleep (G_USEC_PER_SEC / 5);
 	}