INT E0 - DR Multiuser DOS - "A_CATTACH" - CONDITIONALLY ATTACH TO AUX DEVICE
	CL = A7h
Return: AX = status
	    0000h attached
	    FFFFh unable to attach
Desc:	attaches the default auxiliary device to the calling process if it is
	  available
Notes:	does not block if the device is already in use
	also supported by REAL/32
SeeAlso: INT E0/CL=A5h,INT E0/CL=A6h,INT E0/CL=A8h,INT E0/CL=B0h