[AD] load_datafile_object using index

[<spoofer255@xxxxxxxxxx>]

I made a patch that add indexed support for loading single datafile
objects. Here is small example how to use it:

--  cut  --

long index[DATA_COUNT]    //defined in data.h (grabber)
DATAFILE *data;

if (load_datafile_indexed("filename", index) != 0)
  //report error

data = load_datafile_object_indexed("filename", index[DATA_ALIEN]);
if (!data)
  //report error

--  cut --

It's just like normal load_datafile_object, only faster because it
doesn't search for name but fseeks to offset. Any chance it can be
added into 4.3?

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