This function enables presetting of the allocation class of the
 	LD device. This may only be done when no devices are connected.

 	Function:	IO$_LD_CONTROL

 	P1 = allocation class
 	P6 = Subfunction: LDIO$K_SET_ALLOCLASS

 	IOSB word 0 = status

 	Errors returned:

 	SS$_BADPARAM	Allocation class > 255 specified
 	SS$_UNSAFE	There are still connected LD devices
 	SS$_NORMAL	Successful completion