ENVI: avoid generating potentially corrupted .hdr files when opening in update mode; Write 'Arbitrary' instead of 'Unknown' as the projection name for an undefined SRS (#5467, patch by wilsonwaters)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27353 f0d54148-0727-0410-94bb-9a71ac55c965