VT: Add ability to get font requirements

We now have KD_FONT_OP_SET_TALL, but in fact such large fonts cannot be
loaded. No console driver supports tall fonts. Unfortunately, userspace
cannot distinguish the lack of support in the driver from errors in the
font itself. In all cases, EINVAL will be returned.

This patchset adds a separate ioctl to obtain the font parameters
supported by the console driver.

v3:
* Added the use of the in_range macro.
* Squashed the commits that add ioctl to console divers.

v2:
* Instead of the KDFONTOP extension, a new ioctl has been added to
  obtain font information.

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