Patrick Goldstein and others feel that the arrival of United 93 is not only healthy but overdue. And if it’s a less-than-soothing experience for some…well, get used to it. “With the fifth anniversary of Sept. 11 approaching, it is not too soon for movies to offer some unflinching perspective,” Goldstein writes in his “Big Picture” column. “The cold truth is that great art — or at least, as in the case of United 93, sober, thoughtful art — is unruly and impatient. Studios and politicians take polls to see if they are too far out in front of the public. Artists lead with their chin, scrambling headlong out onto ledges or exploring into dark corners. They don’t wait for the audience to form a cozy quorum.”