OSR: preserve authority when importing +init=auth_name:auth_code (e.g. +init=IGNF:LAMB93); also fix the OSR validation to accept the EXTENSION node in DATUM for PROJ4_GRIDS; fix insertion in Spatialite spatial_ref_sys to avoid issues with non-numeric authority codes (auth_srid)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26439 f0d54148-0727-0410-94bb-9a71ac55c965