terminal-util: define yellow as "Khaki3"

Previously used "highlight yellow" was not visible on urxvt, because it
was too light. This color is shown as bold black by urxvt, but at least
it is readable. On other terminals it is shown as various hues of yellow,
pleasant on the eyes and not too bright. The color shown on linux console
could be called orange rather than yellow, but it is still readable.

(I also tried non-highlight "yellow", but it is not as readable.)

This is a follow-up for bb40c125.