Migrate Lidgren.Network to Protobuild

This change migrates Lidgren.Network to Protobuild, which is the last C# project remaining.

Tested on Linux and could build Linux, Windows and WindowsGL platforms.  I guess we'll find out if the other platforms still build once the build server runs (I don't expect too many issues as Lidgren only has one platform-specific file, and all the references are trivial).