Fix zfs-functions packaging bug

This fixes a bug where the generated zfs-functions was being included
along with original zfs-functions.in in the make dist tarball. This
caused an unfortunate series of events during build/packaging that
resulted in the RPM-installed /etc/zfs/zfs-functions listing the
paths as:

ZFS="/usr/local/sbin/zfs"
ZED="/usr/local/sbin/zed"
ZPOOL="/usr/local/sbin/zpool"

When they should have been:

ZFS="/sbin/zfs"
ZED="/sbin/zed"
ZPOOL="/sbin/zpool"

This affects init.d (non-systemd) distros like CentOS 6.

/etc/default/zfs and /etc/zfs/zfs-functions are also used by the
initramfs, so they need to be built even when init.d support is not.
They have been moved to the (new) etc/default and (existing) etc/zfs
source directories, respectively.

Fixes: #9443

Co-authored-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Richard Laager <rlaager@wiktel.com>