kernel/syscons: Add simple font scaling when using KMS drivers.

kernel/syscons: Add simple font scaling when using KMS drivers.

This commit lets the user choose the number of columns for KMS syscons
with a new tunable, kern.kms_columns. If it is not set, the default is
to scale as close as possible to 80x25 using natural numbers as scaling
factors (e.g., on 1680x1050 the default would be a 105x32 char console
using a 16x32 font).

The commit also separates (scaled) character drawing into a new
function, blit_blk(), and does some cleanups.

Submitted-by: dillon