Silences 2 warning messages that appear when using the systemd initrd:
1. "System tainted (var-run-bad)": occurs because `/var/run` isn't a
symlink to `/run`. Fixed by making /run and linking /var/run to it.
2. "Failed to make /usr a mountpoint": occurs because ProtectSystem
defaults to true in the initrd, which makes systemd try to remount
`/usr` as read-only, which doesn't exist in the initrd. Fixed by
linking `/usr/bin` and `/usr/sbin` to the initrd bin directories.
Also moves the `/tmp` creation from the initrd module to make-initrd-ng,
to avoid making an unnecessary `/tmp/.keep`, saving a store path and a
few bytes in the initrd image.
216d98ab97