INT 31 P - CauseWay - "RelMemDOS" - RELEASE CONVENTIONAL MEMORY BLOCK
	AX = FF23h
	DX = initial selector for block
Return: CF clear if successful
	CF set on error
	    AX = DOS error code (see #01680 at INT 21/AH=59h/BX=0000h)
Note:	all descriptors allocated for the block are freed
SeeAlso: AX=FF21h,AX=FF22h,INT 21/AH=4Ah