XC_Core version 6 update.

UnXC_Math.h:
- Enhanced both DoublePlaneDot implementations.
- Linux: Fixed DoublePlaneDot and FLinePlaneIntersectDist.
** (These are used in the relevancy traces)

Created abstract XC_NetConnectionHack class:
- Moved common v436-v440 compatibility macros and functions of XC_Engine and XC_IpDrv into this class.

Included XC_Networking.cpp in sources.
- Contains all the XC_ChannelDownload and XC_NetConnectionHack code.
