INT 21 - Phar Lap 386/DOS-Extender - ALIAS SEGMENT DESCRIPTOR

	AX = 2513h
	BX = segment selector of descriptor in GDT or LDT
	CL = access-rights byte for alias descriptor
	CH = use-type bit (USE16 or USE32) for alias descriptor
Return: CF clear if successful
	    AX = segment selector for created alias
	CF set on error
	    EAX = error code
		08h insufficient memory (can't grow LDT)
		09h invalid segment selector in BX