Makefile:  userpci:  always install headers

Fixes #298

This updates the previous attempt in #333.  It's a hack to avoid
templating `debian/machinekit-dev.install`.
Don't install Cython shared objects into /usr/lib

Another example of the inflexibility of this build system
Hack to get unit tests working from debian packages

This patch modifies the realtime system so that it can run in a
`${DESTDIR}` directory so that unit tests can be run during package
builds, where Machinekit is installed under `${DESTDIR}${prefix}`.
debian/control:  Build and dev requires python-proboscis