dracut: Add ostree-remount

dracut: Add ostree-remount

Linux creates a copy of the soure mount flags when creating a bind
mount; if the source is read-only, then the bind mount is.

The problem is that systemd will remount the rootfs read/write, but
each mount (/home, /var etc.) will still be read-only.  We need to
remount every bind mount except for /usr to read-write too.

This only "worked" with the old ostree-switch-root because it
effectively force mounted the rootfs read-write always, ignoring the
"ro" flag.