set_thread_area sets an entry in the current thread's Thread Local
Storage (TLS) array. The TLS array entry set by set_thread_area
corresponds to the value of u_info->entry_number passed in by the
user. If this value is in bounds, set_thread_area copies the TLS
descriptor pointed to by u_info into the thread's TLS array.
When set_thread_area is passed an entry_number of -1, it uses a
free TLS entry. If set_thread_area finds a free TLS entry, the value of
u_info->entry_number is set upon return to show which entry was