minor fix to scentmap, added more colors to scentmap

Corrected the following line in game::display_scent():

mvwprintz(w_terrain, getmaxy(w_terrain)/2 + y - u.posy, getmaxx(w_terrain)/2 + x - u.posx, sev(sn), "%d",
             sn % 10);

Previously called sev(sn) rather than sev(sn/10); sev() should indicate the color of sn/10, since the number is equal to sn%10, i.e. the remainder. Also added more colors to sev().