[asp.net] Part of fix for bug #649546. A partial rewrite of the web/script resource...

[asp.net] Part of fix for bug #649546. A partial rewrite of the web/script resource handler.

* System.Web/System.Web.Handlers/AssemblyResourceLoader.cs: AssemblyResourceLoader/ScriptResourceHandler must
  always generate the same URL for every assembly+resource pair, or otherwise the browser would re-fetch
  the resource in situations where it is not desired (e.g. during an async request to the server to update
  a panel).
  The generated URLs are not encrypted using real encryption, just hashed.
  In addition, the handler code got modernized a bit.

* System.Web.Extensions/System.Web.UI/ScriptManager.cs: scriptStartupBlock is a 3.5+ thing

* System.Web.Extensions_1.0/Makefile: use the 1.0 Microsoft AJAX library for this compat assembly

* System.Web/System.Web.Util/MachineKeySectionUtils.cs: made GetValidationKey public