Rename 'rtai' featureset to 'rtai.x86'
Add hook into headers-install; RTAI uses for rtai-config script

- Add hook for pkgs wanting to install into linux-headers pkg
- Use it for RTAI to install rtai-config script
Reset abi name to something generic

This only needs to be bumped when a kernel's ABI changes; this
shouldn't happen often
Fix module install hook