The ornamental design for a shower curtainholder, as shown and described.
FIG. 1 is a left side, top and front perspective view of a shower curtain
holder showing my new design;
FIG. 2 is left side elevational view thereof, the side opposite being a
mirror image; and
FIGS. 3 and 4 are respectively top and bottom plan views thereof.
The broken line showing of a fragmentary part of a shower rod and of an
abbreviated part of a shower curtain top is for illustrative purposes only
and forms no part of the claimed design.