* Horschel and Mickelson share lead halfway lead at one under * Five other players one shot off the pace * 68 players yet to finish second round (Adds details, quotes) By Larry Fine ARDMORE, Pennsylvania, June 14 (Reuters) - Phil Mickelson rolled in an 18-foot birdie putt in fading light at the 18th hole to grab a share of the lead in Friday's second round of the U.S. Open when play was suspended due to approaching darkness. ... More...