/etc/default/zfs and /etc/zfs/zfs-functions are now installed by the
makefiles. Continue to install them directly as before, but do not
--fail-missing because of them.
adapted from debian-upstream [0] commit
9a594875114fe186aebba2776b14817ab7f272ae
[0] https://salsa.debian.org/zfsonlinux-team/zfs.git
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>