INT 4B - Virtual DMA Specification - UNLOCK DMA REGION

	AX = 8104h
	DX = flags
	    bit 0: reserved (zero)
	    bit 1: data should be copied out of buffer
	    bits 2-15 reserved (zero)
	ES:DI -> DMA descriptor structure (see #03221,#03222) with region size,
		  physical address, and buffer ID fields set
Return: CF clear if successful
	    DDS physical address field set
	    DDS buffer ID field set (0000h if no buffer allocated)
	CF set on error
	    AL = error code (see #03218)
	    DDS region size field filled wth maximum contiguous length in bytes
Note:	Windows 3.0 does not check whether the region extends beyond the end of
	  a segment
BUG:	Windows 3.0 in enhanced mode does not return a correct code on error
SeeAlso: AX=8103h,AX=8106h