Remove _glapi_check_multithread from the interface exported by the loader to

Remove _glapi_check_multithread from the interface exported by the loader to
the driver.  The loader now takes care of this for the driver.

Remove _glapi_DispatchTSD and give _glapi_Dispatch its semantic (i.e.,
having a NULL value means that the application is multithreaded and
_glapi_get_dispatch must be called).

Gut all of the dispatch override code.  This removes _glapi_RealDispatch,
_glapi_tls_RealDispatch, _glapi_begin_dispatch_override,
_glapi_end_dispatch_override, and _glapi_get_override_dispatch.

Remove _glapi_get_proc_address, _glapi_get_proc_name, _glapi_get_version,
and _glapi_check_table from the loader / driver interface.

Reviewed by: Brian Paul