Corrected the gpio_control_block so that the user_gpio_out signal (#89)

does not pass through an inverter, so that the input can remain
unconnected.  Rewired the existing implementation to use an
alternative gate that has an inverting input so that the
user_gpio_out signal can be left undriven when the GPIOs are in
the management enable state.  This is a simple logic refactoring
and does not change the logic function.  The manual rewiring has
been confirmed by LVS, but at least one GL simulation should be
run to confirm that the logic function remains the same as before.