settings: Add resetSettings() method

Allows the GUI to clear settings.json file and save settings.json.bak file when
GUI "Reset Options" button is pressed or -resetguisettings command line option
is used. (GUI code already backs up and resets the "guisettings.ini" file this
way, so this just makes the same behavior possible for "settings.json")

src/interfaces/node.h

@@ -112,6 +112,10 @@ public:
     //! source, but not being persisted.
     virtual void forceSetting(const std::string& name, const util::SettingsValue& value) = 0;
 
+    //! Clear all settings in <datadir>/settings.json and store a backup of
+    //! previous settings in <datadir>/settings.json.bak.
+    virtual void resetSettings() = 0;
+
     //! Map port.
     virtual void mapPort(bool use_upnp, bool use_natpmp) = 0;