- add disallow_signal() to complement allow_signal(), rather than
   having different subsystems try to do it by hand.

 - add a version of dequeue_signal() which does the necessary locking on
   its own, again to avoid having modules have to care.

 - let allow_signal() to actually allow signals other than
   SIGKILL. Currently they get either converted to SIGKILL or
   silently dropped, according to whether your kernel thread
   happens to have sa_handler set for the signal in question.

   (Barf alert: we do this by just installing a dummy handler)

 - make jffs2 use the cleaned up infrastructure