“Mr. Peabody & Sherman” is the latest postmodern revamp of a modern classic that delves deep into the psyches of characters that were never meant to be analyzed. The original animated TV show is already a pretty subversive treat. Originally broadcast from 1959 to 1964, it simply presents the adventures of a dog and his boy without question. This latest mashup of old and new, from DreamWorks, sets out to answer all those questions about this most unusual dog and boy relationship and turns out to be one sharp and funny movie.