Add on_authplayer callback and 'last_login' to on_joinplayer (#9574)

Replace on_auth_fail callback with more versatile on_authplayer
    Better clarify account login process in Lua API documentation
    Change initial timestamp for newly registered accounts to -1

src/script/cpp_api/s_server.h

@@ -43,7 +43,8 @@ class ScriptApiServer
 	/* auth */
 	bool getAuth(const std::string &playername,
 		std::string *dst_password,
-		std::set<std::string> *dst_privs);
+		std::set<std::string> *dst_privs,
+		s64 *dst_last_login = nullptr);
 	void createAuth(const std::string &playername,
 		const std::string &password);
 	bool setPassword(const std::string &playername,