P_ClosestPointOnLine changes

P_ClosestPointOnLine can now (optionally) take custom coordinates for two points that you want to make up a "line", instead of just actual linedefs.

Complete syntax for P_ClosestPointOnLine as of this branch:

-- syntax for a real linedef (which is already in 2.1.14):
	P_ClosestPointOnLine(x, y, line)

-- syntax for a fake line with vertexes (x1,y1) and (x2,y2):
	P_ClosestPointOnLine(x, y, x1, y1, x2, y2)