If the online is zero, then ibonl() frees the
resources associated with the board or device descriptor
ud. The descriptor cannot be used again after the
ibonl() call.
If the online is nonzero, then all the settings
associated with the descriptor (GPIB address, end-of-string mode, timeout,
etc.) are reset to their 'default' values. The 'default' values are
the settings the descriptor had when it was first obtained with
ibdev() or
ibfind().