The ornamental design for a key chainopener, as shown and described.
FIG. 1 is a front view of a key chain opener;
FIG. 2 is a first side view thereof;
FIG. 3 is a bottom view thereof; and,
FIG. 4 is a perspective view thereof.
The broken lines adjacent to the shaded portions depict unclaimed portions of the claimed design. All other broken lines depict unclaimed environmental subject matter.