Add a _glapi_init function to perform any initialization for the GLAPI layer.

u_current_init will now initialize the TSD keys, instead of trying to create
them when they're first used.

Moved the check to multiple threads to __glDispatchCheckMultithreaded. The
GLAPI layer now just has a _glapi_set_multithread function to tell it to
switch to multi-threaded mode.

Removed glapi/glthread.h, since nothing in it was used anywhere.