Improve PDS dataset identification & fixes image segment offset

[Even Rouault]

Currently, on all the PDS images I could found on the web, the PDS driver doesn't compute correctly the beginning of the image segment that it passes to the RawRasterBand, so we end up reading the header data as image data... This can be easily shown on the 2 datasets available on ​http://download.osgeo.org/gdal/data/pds. This is because we try to read the value of the IMAGE.RECORD_BYTES keyword, whereas I could only find a RECORD_BYTES keyword in files.

By the way, the image data on the GDAL download area in ​http://download.osgeo.org/gdal/data/pds/h2064_0000_gr3.img is clearly corrupted. I could find a correct version at ​http://pds-geosciences.wustl.edu/mex-hrsc-map01/mex-m-hrsc-refdr-mapprojected-v2/mexhrsc_1001/data/2064/h2064_0000_gr3.img, which has the correct size and looks good.

I'll also add code so that images available on ​ftp://pdsimage2.wr.usgs.gov/cdroms/magellan/mg_1103/fl78n018 can be identified as PDS. One must skip the first line which contains a kind of identifier (CCSD3ZF0000100000001NJPL3IF0PDSX00000001 for example). The "PDS_VERSION_ID = PDS3" is only on the following line. It also requires to extand the nasakeywordhandler to accept { "val1", "val2 } as a value for a keyword/value pair.

[Even Rouault]

Fixed in trunk in r14587