Fix some issues in bright colour handling. Bold background doesn't exist

Fix some issues in bright colour handling. Bold background doesn't exist
so there is no reason for tty_check_bg to mess with the BRIGHT flag at
all, ever. Also use aixterm colours for 256-to-16 translation if the
terminal supports them. And there is no reason for tty_colours_bg to
worry about whether the terminal supports them - tty_check_bg has
already taken care of it.