P_LineOpening: set int32 max/min as defaults for opentop, openbottom etc if a...

P_LineOpening: set int32 max/min as defaults for opentop, openbottom etc if a linedef you touched belongs to a polyobjetc. the only thing that really matters in this scenario is the polyobject itself after all!

# Conflicts:
#	src/p_maputl.c