"NMI write protect" not emulated

[jckarter]

This is probably not a priority at all since I doubt any real TOWNS software is affected, but I noticed it while experimenting with some bare-metal code and trying to figure out why it worked in Tsugaru but not real hardware. When 0x80 is written to IO address 0x20, the hardware write-protects RAM at linear addresses [0x8, 0xC) to prevent clobbering the real mode NMI vector (for reference, see Databook section I 3.6, figure I-3-35, page 88). It looks like Tsugaru allows writes to that location even when write protect is enabled.