Since we're assuming IEEE floats these days, there seems little
reason not to do this.  It has the advantage that when the slope is
computed as infinite due to the presence of Inf coordinates, we get
saner behavior than before from line_construct(), and thence also
in some dependent operations such as finding the closest point.

Also fix line_construct() to special-case slope zero.  The previous
coding got the right answer in most cases, but it could compute
C as NaN when the point has Inf coordinates.

Discussion: https://postgr.es/m/CAGf+fX70rWFOk5cd00uMfa__0yP+vtQg5ck7c2Onb-Yczp0URA@mail.gmail.com