Mount handling for --iso (e.g. ISO-on-NTFS-USB)

[wgwoods]

If the ISO provided with --iso isn't on a mount that's listed in /etc/fstab, we won't be able to find it when we reboot.

At the very least, we should refuse to continue with the upgrade in this case.

To handle this cleanly we probably need to poke around in /proc/mounts and/or /proc/self/mountinfo so we can set up the mount after reboot.