Fixed hard link command string for Windows XP
Changed DataFileWriter to use '.ags' extension

This provides consistent data file naming across
target platforms, and also fixes issue of Linux
engine failing to find data files from AGS 3.4.0.2.
Merge pull request #218 from monkey0506/linux_data_file_fix

Fixed Linux game data files not being found by engine
Merge pull request #219 from monkey0506/hardlink_winxp_fix

Fixed hard link command string for Windows XP