Here we use these directives to replace, extend, or clear values in
torrc.defaults and in torrc.

This test runs practracker with a set of 0 thresholds, to make sure
that it enumerates all its values right.  It tries running with an
empty exceptions file, and with an exceptions file that covers
_some_ of the data, and it makes sure that the outputs are as expected.

Includes configuration files to enforce these rules on lib and
common.  Of course, "common" *is* a modularity violation right now,
so these rules aren't as strict as I would like them to be.