Fix debian package builds for xenomai and rtai

- Build Xenomai if libxenomai-dev pkg is installed, not xenomai kernel
- Remove linux-image-xenomai version dependency in linuxcnc-xenomai pkg
- Fix linuxcnc-rtai-kernel package file globs
- Fix things that need to contain 'rtai-kernel' to be picked up properly

Fixes #155