fork(): Follow-up fix to 17b2a3cfc.

* Ingo rightly noticed that the defer_signals counter is reinitialized on
thread's user area creation. Setting the flag THREAD_CREATION_FLAG_DEFER_SIGNALS
indeed gives the expected behavior, deferring signals until undefer_signals() is
called in the child thread. Thanks for the review and fix suggestion.
* Added a simple test showing the values of the defer_signals counter after fork().