Lots of bug fixes to the checkpointing code. The big fix is that you can

Lots of bug fixes to the checkpointing code.  The big fix is that you can
now checkpoint a program that you have checkpoint-restored.  i.e. you run
program X, you checkpoint it, you checkpoint-restore X from the checkpoint,
and then you checkpoint it again.  The issue here is the when a checkpointed
program is restored the checkpoint file is used to map portions of the image
of the restored program.  If you then tried to checkpoint the restored image
the system would overwrite or destroy the original checkpoint file and
the new checkpoint file would have references to the old file (now
non-existant) file.  Any attempt to restore the recursed checkpoint would
result in a seg-fault.  That is now fixed.

* Remove the previous checkpoint file before saving the new one.  If the
  program we are checkpointing happens to be a checkpoint restore from the
  same file then overwriting the file would wind up corrupting the
  image set we are trying to save.

* When checkpointing a program that has been checkpoint-restored do not
  attempt to save the file handles for the vnode representing the
  checkpoint-restored program's own checkpoint file (which is a good chunk
  of its backing store), because this vnode is likely to be destroyed the
  moment we close the handle, since we are likely replacing the previous
  checkpoint file.  Instead, the backing store representing the old
  checkpoint file is copied to the new one.

* Re-checkpointing a program (hitting ^E multiple times) now properly
  replaces the checkpoint file.

* Properly close any file descriptors from the checkpt(1) program itself
  when restoring a checkpointed program, properly replace any file descriptors
  that need replacing.

* Properly replace p_comm[] when restoring a checkpoint file, so checkpointing
  again saves under the same program name.  'ps' output is still wrong,
  though.

TODO LIST:

* Add an iterator to the checkpoint file, accessible via kern.ckptfile,
  so successive checkpoints save to a blah.ckpt.1, blah.ckpt.2, etc,
  rather then always overwriting blah.ckpt (the iterator could be saved
  in the proc structure).

* Add back as a 'feature' the ability for the new checkpoint file to
  reference the old one.  That is, each new checkpoint file would represent
  a delta relative to the old one.  This might be useful when checkpointing
  programs with ever growing data setse so as not to have to copy the
  entire contents of the program to the checkpoint file each time you want
  to make a new checkpoint.  It would be hell on the VM system, but it
  would work.

* Add an option to checkpt(1) so you can checkpoint-restore-enter-gdb all
  in one go, to be able to debug a checkpointed file more easily.

Inspired by: Brook Davis's HPC presentation.  He expressed an interest in
	     possibly porting the checkpoint code so I figure I ought to
	     fix it up.