Fix sandbox usage to instantiate one per job.

I messed up the previous implementation by sharing the same sandbox instance
amongst all submissions. The only visible effect of this is that "max memory"
is max amongst all executions ever. Oops.
Merge branch 'isolate' into debian
debian: Add postrm to remove rc.d links (thanks lintian).
debian: Add build dependency on python-setuptools.
debian: Update 01-use-non-local-dirs patch.