o Make the setting/checking of cancel state atomic with

o Make the setting/checking of cancel state atomic with
  respect to other threads and signal handlers by moving to
  the _thread_critical_enter/exit functions.

o Introduce an static function, testcancel(), that is used by
  the other functions in this module. This allows it to make
  locking assumptions that the top-level functions can't.

o Rework the code flow a bit to reduce indentation levels.

Approved by:	markm/mentor, re/blanket libthr
Reviewed by:	jeff