Fix race condition when multiple jobs are run concurrently.

The default debuild.conf file locations (/etc/dupload.conf and
$HOME/.dupload.conf) are the same for all jobs on a given machine. So
use the -c option to debuild to specify a dupload.conf in the current
directory instead.
Clean up temporary private-key file when done.
Merge pull request #35 from roadrunner2/fixRaceAndLeak

Fix race and leak