Set the right bg color instead of using a dimmed version of the fg
color. The colors are stored in the first 6 bits of the color index.
Thereby the first 3 bits contain the fg and the second 3 bits the
bg color.
The debug message in _handle_esc_seq5() now shows the sequence in
question.
Fixes#495.
sgr0 is currently implemented as a wrapper of sgr(0) which
seems to work fine.
For now we accept but ignore certain sgr sequences like for
example [0;10;1m which is generated by lynx even when using
TERM=linux (I am not sure yet if that's actually valid at all).
