Fix fork() handling on Cygwin (update child thread Id properly and

Fix fork() handling on Cygwin (update child thread Id properly and
update child thread-local storage pointer)

* win32_threads.c (GC_remove_all_threads_but_me): Update comment;
re-hash GC_threads map as thread Win32 Id in the child differs from
that in the parent; rename "id" local variable to "pthread_id".
* win32_threads.c (GC_remove_all_threads_but_me): Call
GC_setspecific (if THREAD_LOCAL_ALLOC) to re-assign Win32-based
thread-local pointer to 'tlfs' (identified by GC_thread_key) of the
current thread.