The federal personnel chief said Thursday that his agency “messed up” the problem-filled launch of the government’s updated job board, and he apologized “to all the applicants who’ve had trouble” finding and applying for jobs for more than three weeks. “If we had the ability to roll back the clock, I sure would do it differently,” John Berry said at a briefing on USAJobs, the federal employment Web site that the government took over from Monster.com 18 months ago.