Use the configuration file path directory for AppSettings File directory

Use the configuration file path directory as the working directory for the file attribute of the appSettings section instead of the current working directory.
(cherry picked from commit b034f7c3e2606f9f27145a8b0e0be9d3ea408479)

We're embedding Mono in our app and the current behavior of AppSettings is to simply pass the value of the file attribute specified in the XML to File.OpenRead(). That causes the current system working directory to be used as the directory for the file, rather than the path of the parent configuration.

Depending on the folder the user executes our binary from the current working directory may or may not be the right path.