It’s the white stocking cap. Hiran Abeysekera wears one when his Hamlet makes that cold but scuttled boat trip from Denmark to England. A man wearing a white stocking cap is not the most eccentric aspect of the “Hamlet” revival that opened Monday at BAM’s Harvey Theatre after a run at London’s National Theatre. The headwear, however, does give us a clue to what Shakespeare’s famous Dane would have been if Truman Capote had ever played the role. …