Prevent a divide-by-zero crash in effect_handler_LIGHT_AREA() due to
operator precedence (thanks to shadowsun for bug report via IRC).
Merge pull request #350 from myshkin/restruct

Fix crash when reading a scroll of light.
Replace FEAT_LIGHTING_* by LIGHTING_*
Ensure a predicate for each SQUARE_* flag
Replace sqinfo_has with predicates