Include ShabbyX/RTAI-style kernel header detection

The ShabbyX/RTAI `rtai-config` script has been updated to support
multiple sets of kernel headers on a system, especially useful for
packaging.  See [this issue][1].

If an rtai.org-style `rtai-config` script is installed, this patch
changes nothing.

If a ShabbyX/RTAI-style `rtai-config` script is installed, this patch
automatically selects a set rtai kernel headers if:

- There is only a single set installed on the system, or
- There is a set matching the running kernel.

And otherwise (no headers found or multiple sets found, an ambiguous
case) fails.

[1]: https://github.com/ShabbyX/RTAI/issues/16