lesspipe: Allow zstd to operate on a symlink

By default zstd refuses to operate on symlinks, so for example
zless /var/crash/vmcore.last.zst failed to view the uncompressed core
file. Add -f to the zstd command line to allow operation on symlinks.

Reviewed by: delphij
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55101