Turn off serial console on boot

[ben at electricembers.net]

> I have some servers with IPMI that allow me to have a serial console. I have 
> setup a serial console config on my servers and it seems to work reasonably 
> well in remote emergencies, but causes serious problems if I ever have to use 
> the real console.
>
> It means that I can't use single user mode from the real console and I can't 
> see most boot errors.
>
> Is there a way I can turn off the console redirection from the physical 
> console interactively on startup?

http://www.freebsd.org/doc/en/books/handbook/serialconsole-setup.html

We put -P in /boot.config so that if a keyboard is detected then the 
screen is the console. No keyboard means serial console.