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/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);