legible colors across all terminal configurations

So far I've been trying to make Teliva follow the default colorscheme of
the terminal, but that easily ends up with illegible color combinations.
New plan: always start with a light background within Teliva. People who
want a dark background will currently need to mess with C sources.

This should somewhat fix #1.
It's still not clear whether the default should be a dark or light
background. While dark background is more common in terminals, I believe
newcomers to terminals will prefer a light background. Then again, I'm
biased since I use a light background in my terminals.

src/lua.c

@@ -1183,7 +1183,7 @@ int main (int argc, char **argv) {
   initscr();
   keypad(stdscr, 1);
   start_color();
-  use_default_colors();
+  assume_default_colors(0, 15);
   draw_menu(L);
   echo();
   s.argc = argc;