Changes in the handling of the format flags for list elements
* Rename P_STAR as P_DISC and P_PLUS as P_SQUARE.
* Delete P_NONE because it was used only as the default flag in init_html_parser
and a list with P_NONE then got bullets, so instead use P_DISC by default (as
per the CSS specification), and P_NO_BULLET for lists with no bullets.
* Use as bullets the characters:
- U+25E6 WHITE BULLET for the circle style;
- U+25AA BLACK SMALL SQUARE (alias square bullet) for the square style;
- U+2022 BULLET for the disc style (default).
Signed-off-by: Fabienne Ducroquet <fabiduc@gmail.com>