add CPL_VSIL_CURL_USE_HEAD config option to disable use of CURL HEAD for other services like mapbox (likely lame python http implementations)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26336 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx