[PATCH] add information on subdataset support to gdal driver metadata

[etourigny]

The following patch implements the DMD_SUBDATASETS metadata item for any driver which supports subdatasets. This information is printed when gdalinfo --frmt <driver> is called.

Just submitting this for review - also would it make sense to apply to 1.9 ?

I have searched for drivers supporting subdatasets with the following command, and rapid inspection of each driver - it might not be 100% accurate.

I have also updated the docs to show that information on the formats list page.

example with hdf4:

$ gdalinfo --format hdf4
Format Details:
  Short Name: HDF4
  Long Name: Hierarchical Data Format Release 4
  Extension: hdf
  Help Topic: frmt_hdf4.html
  Supports: Subdatasets
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