vt(4): Colors are indexed against a console palette, not a VGA palette

Rename vt_generate_vga_palette() to vt_generate_cons_palette() and
change it to build a palette where the color index is the same than in
terminal escape codes, not the VGA index. That's what TCHAR_CREATE()
uses and passes to vt(4).

The main differences between both orders are:
    o  Blue and red are swapped (1 <-> 4)
    o  Yellow and cyan are swapped (3 <-> 6)

The problem remained unnoticed, because the RGB bit indexes passed to
vt_generate_vga_palette() were reversed. This inversion was cancelled
by the colors inversions in the generated palette. For instance, red
(0xff0000) and blue (0x0000ff) have bytes in opposite order, but were
swapped in the palette. But after changing the value of blue (see last
paragraph), the modified color was in fact the red one.

While here, tune the palette to better match console colors and improve
the readability (especially the dark blue).

This is an MFC of r269783 and r269791.