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/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);
 }