[css3-lists] Negativity indicator: minus sign better than hyphen-minus

The CSS3 Lists module Working Draft,
<http://www.w3.org/TR/2002/WD-css3-lists-20021107>, requires the use of a
hyphen-minus (U+002D) to indicate negativity. The Lists module should allow
user agents to use the minus sign (U+2212) instead. Typical hyphen-minus
glyphs are more appropiate for hyphens than for a minus signs.