Bug#36428: MY_MUTEX_INIT_FAST is used before initialization

On some thread implementations, we need a fake mutex attri-
bute as a placeholder, which we define as a global variable,
"my_fast_mutexattr".  Well. that must be initialized before 
used in any mutexes, and the ordering of initializations in 
the API function  my_init()  was wrong.

Now, put my_thread_global_init(), which initializes the attri-
butes that mutexes require.