Update OpenSimulator to use .NET 4.0

This allows region modules to register script functions that have more than a couple of arguments.
It also allows OpenSimulator code to use the feature and syntax additions that came with .NET 4.0
Windows users will need to install Microsoft .NET 4 framework if they have not done so already.
Windows C# Express 2010 also becomes the minimum IDE for build, so runprebuild.bat now generates a 2010 solution and runprebuild2010.bat goes away
Official minimum mono version becomes 2.10.8.1.  OpenSimulator will probably work with 2.8 later but this is not guaranteed.