[System.Configuration]: SaveMode and UserLevel.

[System.Configuration]: SaveMode and UserLevel.

* Configuration.Save() and SaveAs() now honors the
  `ConfigurationSaveMode' argument.

* Check whether the file has been modified prior to saving.

* Fix ConfigurationUserLevel inheritance.

* Implement ConfigurationElement.IsModified().

* New ConfigurationSaveTest.cs test:

  - load a custom section using different parent configuration
    files using ExeConfigurationFileMap and different
    ConfigurationUserLevel values.

  - save configuration using different ConfigurationSaveMode
    arguments and different inherited parent configurations.

* ExeConfigurationFileMapTest.cs: Add error checks for missing
  required filenames.

* New TestUtil.cs helper class:

  - RunWithTempFile() and RunWithTempFiles() helper functions.
  - DotNetVersion, ThisDllName and ThisConfigFileName properties,
    moving all the #ifdef's into one central place.