Pick curve control point farther than in the middle

Seems to get a good point when doing manual tests...

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@11795 b70326c6-7e19-0410-871a-916f4a2858ee
Fix computation of number of quadrants per arc (#2420)

Note: I had to change some tests that expected 3-quadrant curves
      to be accepted as curvey input to rely on round-trip instead

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@11796 b70326c6-7e19-0410-871a-916f4a2858ee