Add -sd option to select subdataset by its number. Slightly reformat the help
message to fit into 80 character wide terminal window.


git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25824 f0d54148-0727-0410-94bb-9a71ac55c965
Add description of new -sd option for gdalbuildvrt.


git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25825 f0d54148-0727-0410-94bb-9a71ac55c965