Choosing a correct working-dir for upgrade-server

[Michael Stone]

When inetd runs upgrade-server/upserv.py, it does so with in the working dir /.

Since the upgrade-server's python modules are not installed in PYTHONPATH, this
choice of working-dir interferes with the module loading that occurs when
re-running python inside fakeroot.

Hardcoding the correct choice of working-dir fixes the observed failure of the
upgrade-server's 'on-demand build download' feature; however, it's probably
worth fixing this for real by either:

 a) packaging upgrade-server so that it properly installs its modules in
    PYTHONPATH or

 b) calculating an appropriate working dir at run time based on a command-line
    argument, environment variable, or based on the location of the python
    script being executed.