boot menu option to disable graphics mode

[Andriy Gapon]

It's long been a wish of mine to have an ability to decide at boot time that a
system should boot in "console-only" mode.  That is, that no graphics/X
applications like e.g. xdm/kdm/gdm are automatically started even when they are
configured to do so.

Here is my attempt at implementing that:
https://gitorious.org/~avg/freebsd/avgbsd/commit/96f7051d63d4286ef6f0196d241e7855338a6ed7?format=patch

All the option does at boot time is setting of 'inhibit_gui' variable for kernel
environment.  I envision that this variable could be properly and gracefully
handled in various startup scripts and/or application startup logic.
But to ensure that the option is always honored I've also added "ultimate
protection" to syscons that prohibits KDSETMODE/KD_GRAPHICS ioctl.

What do you think?
-- 
Andriy Gapon