wow, this is a nasty bug...

wow, this is a nasty bug...

the last part of the snapshot bug is the following:
we snapshot the user threads stack...  when we get a seg fault,
the signal handler is using the same write protected stack...
obviously this is going to cause problems.  luckily there is support
for a special stack for the signal handler.  this checkin switches
the signal handler to run on a different stack than the program stack.