Important change to the NVR code to "fix" the PS/1 and PS/2 systems, plus a cleanup.
All device tables now end with a NULL, not an empty string.
Configuration code now deals better with this.
Machine and Video lookup now return -1 if not found/available, and pc_init() will throw the user into Setup if that is the case.
If for some reason the configured memory size exceeds machine limit, it is now adjusted.
