Adds X-Forwarded-For support to the Unified2 output format

Adds X-Forwarded-For support to the Unified2 output format
 - Added the Unified2 file format related constants
 - Added IPv6 support
 - Two modes of operation with a fall-back to "extra-data" mode if
   "overwrite" mode is not applicable
 - Changed the configuration loading code to handle the new
   configuration structure
 - When creating the packet that fakes the one that generated the alert
   the flow direction wasn't taken into account in overwrite mode
 - Fixed BUG_ON condition