L1B: add support for little-endian LRPT datasets; expose band mask when there are missing scanlines; expose WGS-84 or GRS-80 datum if read from header record (#5645)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27663 f0d54148-0727-0410-94bb-9a71ac55c965