Add a new tunable knob 'MIN_SECT_LABELS' and the necessary stylesheet
support.

This option prevents section labels from being numbered after the third
level.

make FORMATS=ps :

"N.N.N Section Title"
"N.N.N.N really specific topic"
"N.N.N.N.N really-really specific topic"

make MIN_SECT_LABELS=1 FORMATS=ps :

"N.N.N Section Title"
"really specific topic"
"really-really specific topic"

The section titles are still bold, spaced away from the text, and
sized according to their nesting level.