Allow User::isAllowed() to take varargs. "is allowed X or Y" is by far the more...

Allow User::isAllowed() to take varargs.  "is allowed X or Y" is by far the more common multiple permission check in core, so this is now the behaviour of isAllowed( X, Y ); also add isAllowedAll(...) for testing "is allowed X and Y".  Has the nice side effect of adding visibility to a very old function.