Shell exit values must fall into the range of [0-255], so let's honour
this. In practice, the "exit -1" from the old code set an exit value of
255 on most systems, so let's pick that.
Fixes part of bug #14478, patch idea suggested by an anonymous
contributor. Thanks!
Check that tor generates new keys, and overwrites the empty key files.
Test that tor generates new keys when keys are missing (existing
behaviour).
Test that tor does not overwrite key files that already contain data
(existing behaviour).
Tests fixes to bug 13111.
