D.C. United’s return to the MLS playoffs after a five-year hiatus was an inelegant, chaotic and controversial affair tarnished by two own goals, a missed penalty kick and a teenage tantrum. At the end of two odd hours Saturday, United was left with a 1-1 draw against the New York Red Bulls in front of 17,556 at RFK Stadium — the first of two legs in an Eastern Conference semifinal series.