This function enables presetting of the next assigned
 	unitnumber.

 	Function:	IO$_LD_CONTROL

 	P1 = unitnumber to preset
 	P6 = Subfunction: LDIO$K_SET_SEED

 	IOSB word 0 = status

 	Errors returned:

 	SS$_BADPARAM	Unitnumber less than 0 or greater than 9999
 	SS$_NORMAL	Successful completion