Fix debian package builds for xenomai and rtai

- Build Xenomai if libxenomai-dev pkg is installed
  - linux-image-*xeno* is not a build dep
- Remove linux-image-xenomai version dependency in linuxcnc-xenomai pkg
  - xeno-user threads are kversion independent
- Fix linuxcnc-rtai-kernel package file globs
- Fix things needing to be called 'rtai-kernel' to be picked up properly

Fixes #155