If request_control is nonzero, then the board
specified by the board descriptor ud is
made system controller. If request_control
is zero, then the board releases system control.
The system controller has the ability
to assert the REN and IFC lines, and is typically also the
controller-in-charge. Automatic assertion of REN is controlled with
ibconfig().
If the IbcSRE configuration option has been set for the
board descriptor ud
then REN will automatically be asserted when the board becomes
system controller. The configuration option IbcSRE is set by default.
A GPIB bus may not have more than one system controller.