The ornamental design for a keyholder, as shown and described.
1. Key holder
1.1 is a top, right side perspective view of a key holder showing my new design;
1.2 is a top view thereof;
1.3 is a bottom, left side view thereof;
1.4 is a bottom view thereof; and
1.5 is a front view thereof.
The broken line showing of keys in illustrations 1.1 and 1.3 is for the purpose of illustrating environmental structure and forms no part of the claimed design.