color: Assume background is dark if unknown

color: Assume background is dark if unknown

We rely on the COLORFGBG environment variable to tell us whether the
background is dark.  This variable is set by Konsole and rxvt but not
by GNOME Terminal or xterm.  This means we use the wrong set of
colours when GNOME Terminal or xterm is configured with a dark
background.

It appears to me that the dark-background colour palette works better
on a light background than vice versa.  So it is better to assume a
dark background if we cannot find this out from $COLORFGBG.

- Change the initial value of is_dark_bg to 1.
- In set_color_palette(). conditinally set is_dark_bg to 0 with an
  inverted test of the colour.

Signed-off-by: Ben Hutchings <benh@debian.org>