Create subvolumes in init-storage

It turns out that we cannot use `systemd-tmpfiles` to create our Btrfs
subvolumes.  Since the directories we are interested in, specifically
`/var/log` and `/var/tmp` already exist in the rootfs image and are
therefore copied into the mutable filesystem, `systemd-tmpfiles` ignores
them.

To avoid having to explicitly specify the SELinux context for each
subvolume created on the persistent filesystem, `init-storage` now
executes `setfiles` to set the appropriate labels.

build-rootfs.sh

@@ -68,7 +68,6 @@ if [ -f /mnt/gentoo/etc/udev/hwdb.bin ]; then
 fi
 
 rm -f /mnt/gentoo/lib/tmpfiles.d/provision.conf
-sed -i 's:d /var/log :v /var/log :' /mnt/gentoo/lib/tmpfiles.d/var.conf
 systemd-tmpfiles --root=/mnt/gentoo -E --exclude-prefix=/var --create
 
 systemctl preset-all --root=/mnt/gentoo