Shutdown sometimes hangs when using "changes" file

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Sometimes the AppPot image hangs during shutdown, just after INIT has finished 
killing all the tasks, and must be halted from `uml_mconsole`.

My guess is that INIT (or Debian's `/etc/rc` script) is using the 
atime/ctime/mtime of some file or directory as a reference timestamp; this 
access time is changed by the `apppot-snap` script when merging the changes, 
and thus confuses INIT.  Likely, the reference timestamp is set in the past (to 
the time of creation of the "changes" file), hence INIT is just waiting for 
time to pass.

If this guess is confirmed, we should find out what this reference 
file/directory is, and exclude it from `apppot-snap` operations, modifying 
`/var/lib/apppot/apppot-snap.exclude`.

Original issue reported on code.google.com by riccardo.murri@gmail.com on 1 Mar 2012 at 7:20