The ornamental design for a keyholder, as shown and described.
FIG. 1 is a perspective view of the key holder;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a right elevation view thereof;
FIG. 5 is a left elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line showing of a selectively deployable key are included for the purpose of illustrating environment and form no part of the claimed design.