(PUP-1591) Allow elevate.exe to run against .NET 4

 - Various Windows editions ship with various editions of the .NET
   Framework.  This can also vary by how recently Windows Updates have
   run, and local AD policy.
 - According to Corflags.exe, elevate.exe has the following properties
   CorFlags.exe .\elevate.exe
   Microsoft (R) .NET Framework CorFlags Conversion Tool.  Version  4.0.30319.33440
   Copyright (c) Microsoft Corporation.  All rights reserved.

   Version   : v2.0.50727
   CLR Header: 2.5
   PE        : PE32
   CorFlags  : 0x9
   ILONLY    : 1
   32BITREQ  : 0
   32BITPREF : 0
   Signed    : 1
 - .NET 3.5, 3.0 and 2.0.50727 all use the .NET 2.0 CLR, so we can allow
   elevate.exe to run on any of these.
 - After some manual testing, it was determined that the .NET 4.0 CLR
   also works correctly for elevate.exe, and so we can override the .NET
   runtime selection by providing a side ar elevate.exe.config file.