1. We require that the user has defined (either
   in the environment or in one of the pbuilderrc
   files), two devices:
   	BASEDEV
   	  This is the device where the persistent
   	  ext2 filesystem containing the basic build
   	  file system lies (what is usually kept
   	  in base.tgz). This file system should be
   	  e2fsck -f clean. create/update/clean go here.
   	COWDEV
   	  This is the device to which the writes
   	  will actually go during the build/login/execute.

2. For create/update/clean, we just mount BASEDEV in the appropriate
   place and run pbuilder create/update/clean --no-tgz. We run an
   e2fsck -f at the end.

3. For all other operations:
        a. we create a new device TOTALDEV consisting of BASEDEV
           extended by zeroes by the size of COWDEV.
        d. we create a snapshot version WORKDEV of TOTALDEV with
           the writes going to COWDEV.
        e. we resize2fs the filesystem WORKDEV. This 
           step could be *slow*.
        f. we mount WORKDEV in the appropriate place and run
           pbuilder with the appropriate command.
        g. on exit or error we must unmount WORKDEV and 
           remove WORKDEV and TOTALDEV.