- The PM code currently must signal each kernel thread when suspending, and
  each thread must call refrigerator() to stop itself. This patch adds 
  support for this to migration_thread, which allows suspend states to work
  on an SMP-enabled kernel (though not necessarily an SMP machine).

- Note I do not know why the process freezing code was designed in such a 
  way. One would think we could do it without having to call each thread
  individually, and fix up the threads that need special work individually..