WASHINGTON — Early in his speech at the Libertarian Party’s national convention on Saturday, Donald Trump told the party’s delegates bluntly that they should nominate him as its candidate for president. He was vigorously booed. When the jeers died down, Trump, visibly frustrated with the rowdy reception he had received ever since taking the stage, dug in and went a step further, seeming to insult the very group that had invited him. “Only do that if you want to win,” he said of nominating him.