post: Re-instate systemd ProtectHome/ProtectSystem and PrivateTmp
This reverts https://pagure.io/fedora-atomic/c/6a8ee9a65491d5a37c9177a6d8a709a4ecb3baf7?branch=master
I've tested that a rebase of F26AH works with the latest systemd there; for
example it looks like `chronyd.service` uses `PrivateTmp=yes` and
`ProtectSystem=full`, and it's running fine.