RK11,RL11: no proper NXM check in 18bit systems

[wfjm]

No NXM error is generated when a UNIBUS device DMA transfer reaches the top of memory in 18 bit addressing. Seen originally for RK11, but RL11 and DEUNA are also affected.

In normal operation is minor non-conformity is not relevant because operating systems never setup DMA transfers to/from non-existing memory. However, some highly space optimized crash dump routines use this to detect end-of-memory and might endless loop. Also diagnostics, like zrkkf2, might test this border case and fail.

originally filed as issue w11a_V0.50-8 and TCK-035 dated 2009-05-15