vt(4): Use default VGA palette

Before this change, the VGA palette was configured to match the shell
palette (e.g. color #1 was red). There was one glitch early in boot when
the vt(4)'s VGA palette was loaded: the loader's logo would switch from
red to blue. Likewise for the "Booting..." message switching from blue
to red. That's because the loader's logo was drawed with the default VGA
palette where a few colors are swapped compared to the shell palette
(e.g. blue <-> red).

This change configures the default VGA palette during initialization and
converts input's colors from shell to VGA palette index.

There should be no visible changes, except the loader's logo which will
keep its original color.

Reviewed by: eadler