The Linux Lab Project GPIB driver does not provide an equivalent to the
ibdev() function that finds a GPIB device by address.  That makes the 
Linux Lab Project driver the only one that does not provide such an
interface, so it is easier just to provide one for it.  This is handled
by adding to /etc/gpib.conf the contents of the file gpib.conf_addon
in this directory, so that there is a way to find a given GPIB device
by its primary address.