The ornamental design for a corkscrew, as shown and described.
FIG. 1 is a front perspective view of a corkscrew showing our new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a left side elevational view thereof, the right side being a mirror image thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is a cross-sectional view thereof, taken along line 7—7 of FIG. 5, for reference and to facilitate an understanding of the features shown in other figures.