Add a sysctl to allow disabling resetting the OF syscons.

On some machines (G5 with lots of RAM), entering OF sometimes causes the
machine to hang.  Once the machine is booted, currently the only entry point
into OF is through resetting the framebuffer on mode switch on these machines.
Disabling this allows the machine to stay up at the expense of less usable
consoles after X is started.

MFC after:	Never, this is only a hack