Ignore ECHILD in UnixEventPort::ChildSet::checkExits

If no child process has changed state, waitpid() returns zero. But if there
are no more child processes at all, it returns -1 and sets errno to ECHILD.
Since checkExits calls waitpid() in a loop, it must ignore ECHILD.

+Unit test modified to exhibit the unexpected behaviour.

c++/src/kj/async-unix-test.c++

@@ -707,12 +707,7 @@ TEST(AsyncUnixTest, ChildProcess) {
   KJ_DEFER(KJ_SYSCALL(sigprocmask(SIG_SETMASK, &oldsigs, nullptr)) { break; });
 
   TestChild child1(port, 123);
-  TestChild child2(port, 234);
-  TestChild child3(port, 345);
-
   KJ_EXPECT(!child1.promise.poll(waitScope));
-  KJ_EXPECT(!child2.promise.poll(waitScope));
-  KJ_EXPECT(!child3.promise.poll(waitScope));
 
   child1.kill(SIGTERM);
 
@@ -722,6 +717,9 @@ TEST(AsyncUnixTest, ChildProcess) {
     KJ_EXPECT(WEXITSTATUS(status) == 123);
   }
 
+  TestChild child2(port, 234);
+  TestChild child3(port, 345);
+
   KJ_EXPECT(!child2.promise.poll(waitScope));
   KJ_EXPECT(!child3.promise.poll(waitScope));
 

c++/src/kj/async-unix.c++

@@ -145,7 +145,12 @@ void UnixEventPort::ChildSet::checkExits() {
   for (;;) {
     int status;
     pid_t pid;
-    KJ_SYSCALL(pid = waitpid(-1, &status, WNOHANG));
+    KJ_SYSCALL_HANDLE_ERRORS(pid = waitpid(-1, &status, WNOHANG)) {
+      case ECHILD:
+        return;
+      default:
+        KJ_FAIL_SYSCALL("waitpid()", error);
+    }
     if (pid == 0) break;
 
     auto iter = waiters.find(pid);