loader: use shadow framebuffer
Make sure we do not read from FB in loader, unless we can not allocate shadow buffer. This should improve performance for many systems.
Testoing done: build/install/boot, my own test systems do not show great difference, but as with related kernel gfx_private work, there are systems to win from it. Also had positive feedback from FreeBSD specific patch (FreeBSD version is slightly different because of tem and teken callback differences).