Embarrassed by a series of shocking hazing complaints — including one in which a rookie claimed he was sexually abused — the FDNY has quietly overhauled its anti-hazing policy, The Post has learned. Without any public announcement, Fire Commissioner Daniel Nigro on March 21 issued a new five-page policy. The internal document, obtained by The Post, lists 17 prohibited behaviors, including several recently exposed in a series of Post reports. For the first time, the FDNY says it will refer any conduct considered a “possible crime” to the city Department of Investigation. It also says every officer and employee has an “obligation” to report such acts.