_FLUSH (5Fh) unmaps drives when the "flush and invalidate" flag is set

[Konamiman]

The "Flush disk buffers" function call (_FLUSH, 5Fh) executes the ZAP_ALL routine when it's invoked with D=FFh ("flush and invalidate" flag); this routine destroys all unit descriptors, so the next access to the corresponding drives rebuilds them. While that made sense in MSX-DOS, in Nextor this causes a new automatic mapping of the drives to be performed (which often results in the drive being left unmapped). The unit descriptor destruction should happen only for drives mapped to MSX-DOS kernels.