mount: add support for fsck

mount: add support for fsck

- Dirty/clean status handling
- Write back superblocks when we go clean but not if we are clean (or bad)
- Add full checks so remount ro works for all cases

This allows you to fsck -a `prtroot` in /etc/rc to recover the system if
needed. Before rebooting you need to remount filesystems r/o or unmount them.

This last bit wants wiring into reboot so it does a runlevel change to init
unless -f is used. We can then umount everything nicely (by adding umount -a)
and remount the rootfs r/o as well as doing a killall etc when we shutdown.