Support transient /etc

Support transient /etc

If the `prepare-root.conf` file contains:
```
[etc]
transient=yes
```

Then during prepare-root, an overlayfs is mounted as /etc, with the
upper dir being in /run. If composefs is used, the lower dir is
`usr/etc` from the composefs image , or it is the deployed
`$deploydir/usr/etc`.

Note that for this to work with selinux, the commit must have been
built with OSTREE_REPO_COMMIT_MODIFIER_FLAGS_USRETC_AS_ETC. Otherwise
the lowerdir (/usr/etc) will have the wrong selinux contexts for the
final location of the mount (/etc).

We also set the transient-etc key in the ostree-booted file, pointing it
to the directory that is used for the overlayfs.

There are some additional work happening in ostree-remount, mostly
related to selinux (as this needs to happen post selinux policy
load):

 * Recent versions of selinux-poliy have issues with the overlayfs
   mount being kernel_t, and that is not allowed to manage files as
   needed. This is fixed in
   https://github.com/fedora-selinux/selinux-policy/pull/1893

 * Any /etc files created in the initramfs will not be labeled,
   because the selinux policy has not been loaded. In addition, the
   upper dir is on a tmpfs, and any manually set xattr-based selinux
   labels on those are reset during policy load. To work around this
   ostree-remount will relabel all files on /etc that have
   corresponding files in overlayfs upper dir.

 * During early boot, systemd mounts /run/machine-id on top of
   /etc/machine-id (as /etc is readonly). Later during boot, when etc
   is readwrite, systemd-machine-id-commit.service will remove the
   mount and update the real file under it with the right content. To
   ensure that this keeps working, we need to ensure that when we
   relabel /etc/machine-id we relabel the real (covered) file, not the
   temporary bind-mount.

 * ostree-remount no longer needs to remount /etc read-only in the
   transient-etc case.

Signed-off-by: Alexander Larsson <alexl@redhat.com>