Convert all remaining geometric operators to new fmgr style. This

Convert all remaining geometric operators to new fmgr style.  This
allows fixing problems with operators that expected to be able to
return a NULL, such as the '#' line-segment-intersection operator
that tried to return NULL when the two segments don't intersect.
(See, eg, bug report from 1-Nov-99 on pghackers.)  Fix some other
bugs in passing, such as backwards comparison in path_distance().