Improve pool seeding during startup:

* When starting up, we no longer rely just on the standard
  system RNG to generate entropy: we attempt to squeeze some
  from the execution state, and to recover any entropy that
  we had previously stored.

* When shutting down, if sufficient entropy has been accumulated
  attempt to store it for future use.

src/ripple/core/Config.h

@@ -124,6 +124,9 @@ public:
     /** Returns the full path and filename of the debug log file. */
     boost::filesystem::path getDebugLogFile () const;
 
+    /** Returns the full path and filename of the entropy seed file. */
+    boost::filesystem::path getEntropyFile () const;
+
     // DEPRECATED
     boost::filesystem::path CONFIG_FILE; // used by UniqueNodeList
 private: