The ornamental design for a corkscrew, as shown and described.
1. Corkscrew
1.1: Perspective
1.2: Front
1.3: Bottom
1.4: Top
1.5: Left
1.6: Right
1.7: Unfolded
1) The name of the product incorporating the design is “corkscrew”; 2) The use of the product incorporating the design is “to draw corks from bottles with a foldable pointed spiral piece of metal and a notched lever allowing to take support on the top of the bottle's neck; to be electrically recharged via the round-shaped terminal located on the right end of the handle (view 1.6)”; 3) The essential features of the design are the shape and pattern of the corkscrew; and 4) The view 1.1best shows the essential features of the design (perspective view).