<em>'Parental Guidance'</em> drags. It's very much a one-man band as the amusing Billy Crystal stops this from dropping to deeply low depths, he has a decent amount of good gags and is full of sharp wit. Bette Midler is a (not so) close second, she bounces off Crystal's comedy well. All of the cast that play their family are entirely forgettable, almost annoying in fact. The premise, though it may conclude…