[GLX] Fix-up TSD context key initialization code

- Do some renaming to reduce confusion:

  threadDestroyKey -> tsdContextKey
  threadDestroyKeyInitialized -> tsdContextKeyInitialized
  threadInitOnceControl -> threadCreateTSDContextOnceControl
  ThreadInitOnce -> ThreadCreateTSDContextOnce

- Delete __glXInitThreads().  Instead the TSD context key is initialized
  when TrackCurrentContext() is first called.

- XGLVRegisterCloseDisplayCallback(DisplayClosed) is called by
  __glXInit() now instead of ThreadInitOnce().