Support for more video modes: accept mode names like MODE_<NUMBER> where

Support for more video modes: accept mode names like MODE_<NUMBER> where
<NUMBER> is the video mode number from the vidcontrol -i mode output.
MODE_13 is 320x200, for example...

Change the default geometry of raster modes: calculate rows and columns
using the font width and height. This results in the row * col geometry
being as large as possible if no geometry is specified by the user. If
no font is specified the current font's height is used. The old
vidcontrol used a default geometry of 80x25 for raster modes.

Remove restrictions on the range of background colors for graphics modes.

Implement a cleaner revert mechanism: in case something goes wrong we
restore the previous settings and give a message telling what exactly
went wrong. Unfortunately, there are several parameters that cannot
(yet) be saved and restored due to restrictions in the syscons driver:
screen saver timeout, cursor type, mouse character, mouse show/hide
state, vty switching on/off state, history buffer size and history
buffer contents are not saved because the syscons driver doesn't provide
ioctl()s to read those parameters. Font maps are not restored because
the syscons driver won't let us load a font map without setting it.

Submitted-by: Sascha Wildner <saw@online.de>