Process all systemd services through the systemd scriptlets

This patch ensures that all systemd services are processed through the
systemd scriptlets, so that services are properly configured per the
preset file installed by the package.

Without this, zfs.target is set, but none of the services are enabled per
the preset file, meaning automounting filesystems and such won't work
out of the box.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Neal Gompa <ngompa13@gmail.com>
Closes #5356