The ornamental design for a knifeholder, as shown and described.
FIG. 1 is a perspective right side view in which knives with which the inventive holder may be used and an illustrative base for the inventive knife holder are shown in dashed line to show the context of the environment in which the inventive knife holder is used and form no part of the claimed design;
FIG. 2 is a perspective front view of the knife holder of FIG. 1;
FIG. 3 is a top plan view of the knife holder of FIG. 1;
FIG. 4 is a front elevation view of the knife holder of FIG. 1;
FIG. 5 is a right side elevation view of the knife holder of FIG. 1;
FIG. 6 is a left side elevation view of the knife holder of FIG. 1;
FIG. 7 is a rear elevation view of the knife holder of FIG. 1; and,
FIG. 8 is a bottom plan view of the knife holder of FIG. 1.