BUG/MINOR: checks: external checks shouldn't wait for timeout to return the result

When the child process terminates, it should wake up the associated task to
process the result immediately, otherwise it will be available only when the
task expires.

This fix is specific to the 1.6 branch.

src/checks.c

@@ -1519,6 +1519,7 @@ static void pid_list_expire(pid_t pid, int status)
 			elem->t->expire = now_ms;
 			elem->status = status;
 			elem->exited = 1;
+			task_wakeup(elem->t, TASK_WOKEN_IO);
 			return;
 		}
 	}