INT 21 - Phar Lap 386/DOS-Extender VMM v3.0 - READ/WRITE LDT DESCRIPTOR

	AX = 2531h
	BX = segment selector
	ECX = direction (00h read, 01h write)
	DS:EDX -> 8-byte buffer for descriptor contents
Return: CF clear if successful
	CF set on error
	    EAX = error code
		81h invalid selector
		82h DPMI running, or not a code or data segment