Adjust vesafb graphics mode (bug #145).

The problem seems that some (server) gfx cards do not support >64K
colors when in VESA mode. Since we don't need more than that we are going to use
vga=791 from now on (like sysresccd does) instead of vga=792.
Add update mode for sysresccd unpack script.
Bump systemrescuecd version to 3.3.0