Expose more properties of the connection validator context in MQTTnet.Server. Migrate return code to reason code.

Source/MQTTnet.Server/Configuration/PathHelper.cs

@@ -0,0 +1,24 @@
+using System;
+using System.IO;
+
+namespace MQTTnet.Server.Configuration
+{
+    public static class PathHelper
+    {
+        public static string ExpandPath(string path)
+        {
+            if (path == null)
+            {
+                return null;
+            }
+
+            var uri = new Uri(path, UriKind.RelativeOrAbsolute);
+            if (!uri.IsAbsoluteUri)
+            {
+                return Path.Combine(AppDomain.CurrentDomain.BaseDirectory, path);
+            }
+
+            return path;
+        }
+    }
+}