kernel autodetect: rework RTAI detection

/dev/rtai_shm isnt used anymore, and it would have required the RTAI
modules to be insert to reliably detect RTAI.

Instead, use differences in the /proc/ipipe directory to tell RTAI from
Xenomai:  Xenomai has /proc/ipipe/Xenomai and /dev/rtheap both of which
RTAI doesnt have; so use this difference for now

This is going to remain heuristic.