Fix for #1890 and #1959

This bug happens under Windows & Embedded server
Reason is that pthread_self() always returns NULL in this case.
This confuses thr_lock function and it doesn't stop
thread inserting in the write-locked table.

Global problem is that there's no way under Windows to get
unique thread handle for working thread.
Monty made a workaround for server - we store the thread's handle
we get when we create thread in the thread-specific variable.
This doesn't work with the embedded library for we don't control
thread creation there.

I added code that sets CurrentThreadId as the pthread_self
for the embedded library.
It seems to solve problem because it's unique and we don't use
pthread_self as a parameter for thread functions in embedded library.

mysys/my_thr_init.c

@@ -157,6 +157,9 @@ my_bool my_thread_init(void)
   tmp= &THR_KEY_mysys;
 #endif
   tmp->id= ++thread_id;
+#if defined(__WIN__) && defined(EMBEDDED_LIBRARY)
+  tmp->thread_self= (pthread_t)getpid();
+#endif
   pthread_mutex_init(&tmp->mutex,MY_MUTEX_INIT_FAST);
   pthread_cond_init(&tmp->suspend, NULL);
   tmp->init= 1;