Cygwin: fork: attach child not before success

Do not attach to the child before it was successfully initialized, or we
would need more sophisticated cleanup on child initialization failure,
like suppressing SIGCHILD delivery with multiple threads ("waitproc")
involved.

Improves "Cygwin: fork: Remember child not before success.",
commit f03ea8e1c5, which leads to fork
problems if cygserver is running:

https://cygwin.com/ml/cygwin-patches/2019-q2/msg00155.html

winsup/cygwin/fork.cc

@@ -421,14 +421,6 @@ frok::parent (volatile char * volatile stack_here)
       this_errno = EAGAIN;
 #ifdef DEBUGGING0
       error ("child remember failed");
-#endif
-      goto cleanup;
-    }
-  if (!child.reattach ())
-    {
-      this_errno = EAGAIN;
-#ifdef DEBUGGING0
-      error ("child reattach failed");
 #endif
       goto cleanup;
     }
@@ -516,6 +508,17 @@ frok::parent (volatile char * volatile stack_here)
 	}
     }
 
+  /* Do not attach to the child before it has successfully initialized.
+     Otherwise we may wait forever, or deliver an orphan SIGCHILD. */
+  if (!child.reattach ())
+    {
+      this_errno = EAGAIN;
+#ifdef DEBUGGING0
+      error ("child reattach failed");
+#endif
+      goto cleanup;
+    }
+
   /* Finally start the child up. */
   resume_child (forker_finished);