LIBKML: implement ATC 15 (linestring has >= 2 points and ATC 16 (linearring has >= 4 points)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26924 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx