Peter Debruge Chief Film CriticIf you’ve ever seen Mike Birbiglia before, whether on stage or screen (or a couple months back, filling in for Jimmy Kimmel), then “The Old Man and the Pool” feels like catching up with an old friend — albeit one with a lot more health problems than you.To be clear: In his latest monologue, Mike Birbiglia is not the old man. At age 44, it’s a little early for that. But the lower-middle-age comic is plenty worried about his health.