Cygwin: pinfo: stop remember doing reattach

During fork, the child process requires the process table to be
initialized for fixup_shms_after_fork, while still allowing subsequent
dlls.load_after_fork to fail silently (for when the "forkable" hardlinks
are not created yet).
pinfo::remember not performing reattach anymore requires explicit
pinfo::reattach now where appropriate.

Prepares to improve "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

@@ -421,6 +421,14 @@ 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;
     }

winsup/cygwin/sigproc.cc

@@ -216,9 +216,7 @@ proc_subproc (DWORD what, uintptr_t val)
 	  vchild->process_state |= PID_INITIALIZING;
 	  vchild->ppid = what == PROC_DETACHED_CHILD ? 1 : myself->pid;	/* always set last */
 	}
-      if (what == PROC_DETACHED_CHILD)
-	break;
-      /* fall through intentionally */
+      break;
 
     case PROC_REATTACH_CHILD:
       procs[nprocs] = vchild;
@@ -873,7 +871,8 @@ void
 child_info_spawn::wait_for_myself ()
 {
   postfork (myself);
-  myself.remember (false);
+  if (myself.remember (false))
+    myself.reattach ();
   WaitForSingleObject (ev, INFINITE);
 }
 

winsup/cygwin/spawn.cc

@@ -779,7 +779,9 @@ child_info_spawn::worker (const char *prog_arg, const char *const *argv,
 	  child->start_time = time (NULL); /* Register child's starting time. */
 	  child->nice = myself->nice;
 	  postfork (child);
-	  if (!child.remember (mode == _P_DETACH))
+	  if (mode == _P_DETACH
+	      ? !child.remember (true)
+	      : !(child.remember (false) && child.reattach ()))
 	    {
 	      /* FIXME: Child in strange state now */
 	      CloseHandle (pi.hProcess);