wallet: rejig to avoid prompting in wallet2

wallet2 is a library, and should not prompt for stdin. Instead,
pass a function so simplewallet can prompt on stdin, and a GUI
might display a window, etc.

src/common/password.h

@@ -82,7 +82,7 @@ namespace tools
        \return The username and password, or boost::none if
          `password_container::prompt` fails.
      */
-    static boost::optional<login> parse(std::string&& userpass, bool verify, const char* message = "Password");
+    static boost::optional<login> parse(std::string&& userpass, bool verify, const std::function<boost::optional<password_container>(bool)> &prompt);
 
     login(const login&) = delete;
     login(login&&) = default;