introduce OGRLayer::FindFieldIndex() / OGR_L_FindFieldIndex() to lookup potentially laundered field names

From: Juergen E. Fischer <jef@norbit.de>

git-svn-id: https://svn.osgeo.org/gdal/trunk@26572 f0d54148-0727-0410-94bb-9a71ac55c965
make relaxed lookup optional and add a switch -relaxedFieldNameMatch to ogr2ogr to allow it

From: Juergen E. Fischer <jef@norbit.de>

git-svn-id: https://svn.osgeo.org/gdal/trunk@26573 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx