container: T9456: use kernel overlayfs unless storage sits on overlayfs

Since T5003 every system was configured to mount container storage via
fuse-overlayfs. This is only needed on live boot, where the storage sits
on the root overlayfs and podman refuses native overlay. Installed
systems keep it on the persistence partition and paid for a userspace
FUSE process per container plus a round trip on every file operation.

The mount program is now only configured when the storage is backed by
overlayfs, otherwise the kernel overlay driver is used.

Existing storage keeps using fuse-overlayfs: podman records that a mount
program was used, as its layers store whiteouts in a format the kernel
driver does not understand. Moving such a system to kernel overlayfs
requires resetting the container storage and re-pulling all images.

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