libc - Fix some recursion issues during thread teardown

libc - Fix some recursion issues during thread teardown

* thread destructors called by sophisticated pthreaded programs such
  as firefox can wind up allocating/freeing space multiple times
  AFTER nmalloc's destructor is run.

  This can leave the mtmagazine in a weird state because it's destructor
  fails to NULL-out tp->mags[i].loaded and tp->mags[i].prev.

* Properly NULL out tp->mags[i].{loaded,prev} in the destructor.

* Lockout mtmagazine use while the mtmagazine is being initialized or
  destroyed.

* Permanently lockout mtmagazine use after its destructor has been run.

* Instead of trying to initialize the mtmagazine on the first free, which
  might not occur until the destrutor is run (causing pthreads to complain
  about destructors being left hanging), have libthread_xu call a new
  function _nmalloc_thr_init() during thread creation and do all the slab
  initialization there.