Section handlers deprecated implementation

[obones]

While working on Linux unit tests (#304), I came across the way the xslFiles section is accessed.
The current code is using ConfigurationManager.GetSection("xslFiles") and casts it directly to an IList instance. Unfortunately, for a reason that I could not identify, this always returns null under Linux/Mono.
However, using ConfigurationManager.OpenExeConfiguration gives me a valid filePath along with a non null value for the GetSection code.
Unfortunately, the returned object is a DefaultSection instance that does not cast to IList at all.

This led me to review the code that "reads" this from the .config file and I discovered that it is implemented via the XslFilesSectionHandler class. It does so by implementing the IConfigurationSectionHandler interface which has been deprecated for a long time now.
Changing the code to follow today's standards allows to get an instance of XslFilesSectionHandler returned by GetSection and as such access any code on it that could return a list of strings as expected by users of this section.

This pull request implements the required changes and with those, I have solved failed tests 2, 3 and 4.

Note that there is another location where GetSection is called, inside CruiseServerFactory.CreateLocal, along with the ServerConfigurationHandler section handler that implements IConfigurationSectionHandler. I believe the same set of changes is required there as well.

There is one last usage of IConfigurationSectionHandler, and it's in CruiseControlConfigSectionHandler. But I don't understand its usage as its Create method returns null in all cases. Maybe it should just be removed.