Prevent passing of non-POD types to POD-only interfaces:

This tidies up the code that produces random numbers to conform
to programming best practices and reduce dependencies.

* Use std::random_device instead of platform-specific code
* Remove RandomNumbers class and use free functions instead

src/ripple/app/main/Main.cpp

@@ -218,11 +218,8 @@ int run (int argc, char** argv)
     po::positional_options_description p;
     p.add ("parameters", -1);
 
-    if (! RandomNumbers::getInstance ().initialize ())
-    {
-        std::cerr << "Unable to add system entropy" << std::endl;
-        iResult = 2;
-    }
+    // Seed the RNG early
+    add_entropy ();
 
     if (!iResult)
     {