Fix shutdown issues

When shutting down, archiso is unable to unmount some locations:

[FAILED] Failed unmounting Temporar…/etc/pacman.d/gnupg directory.
[  OK  ] Unmounted /mnt/arch-boxes.
[  OK  ] Unmounted /mnt/scratch-disk.
[FAILED] Failed unmounting /run/archiso/bootmnt.
[  OK  ] Unmounted /run/archiso/cowspace.
[  OK  ] Unmounted /run/archiso/sfs/airootfs.
[  OK  ] Unmounted Temporary Directory (/tmp).
[  OK  ] Stopped target Swap.
[  OK  ] Unmounted /var/cache/pacman/pkg.
[  OK  ] Stopped target Local File Systems (Pre).
[  OK  ] Reached target Unmount All Filesystems.
         Stopping Monitoring of LVM…meventd or progress polling...
[  OK  ] Stopped Create Static Device Nodes in /dev.
[  OK  ] Stopped Create System Users.
[  OK  ] Stopped Remount Root and Kernel File Systems.
[  OK  ] Stopped Monitoring of LVM2… dmeventd or progress polling.
         Stopping LVM2 metadata daemon...
[  OK  ] Stopped LVM2 metadata daemon.
[  OK  ] Reached target Shutdown.
[  OK  ] Reached target Final Step.
[  OK  ] Closed LVM2 metadata daemon socket.
[  OK  ] Finished Power-Off.
[  OK  ] Reached target Power-Off.
umount: can't unmount /oldroot/sys/kernel/tracing
/oldroot/sys/kernel/security
/oldroot/sys/kernel/debug
/oldroot/sys/kernel/config
/oldroot/sys/fs/pstore
/oldroot/sys/fs/cgroup/unified
/oldroot/sys/fs/cgroup/systemd
/oldroot/sys/fs/cgroup/rdma
/oldroot/sys/fs/cgroup/pids
/oldroot/sys/fs/cgroup/perf_event
/oldroot/sys/fs/cgroup/net_cls,net_prio
/oldroot/sys/fs/cgroup/memory
/oldroot/sys/fs/cgroup/hugetlb
/oldroot/sys/fs/cgroup/freezer
/oldroot/sys/fs/cgroup/devices
/oldroot/sys/fs/cgroup/cpuset
/oldroot/sys/fs/cgroup/cpu,cpuacct
/oldroot/sys/fs/cgroup/blkio
/oldroot/sys/fs/cgroup
/oldroot/sys/fs/bpf
/oldroot/sys
/oldroot/proc
/oldroot/dev/shm
/oldroot/dev/pts
/oldroot/dev/mqueue
/oldroot/dev/hugepages
/oldroot/dev
/oldroot: No such file or directory
umount: can't unmount /dev/loop0: Invalid argument
umount: can't unmount /oldrun/archiso/cowspace: Invalid argument
umount: can't unmount /oldrun/archiso/bootmnt: Device or resource busy
[  195.978961] reboot: Power down

While this is technically not a problem (I hope) it goes without saying that it would be more pretty if archiso would not fail to do its tasks during shutdown.

Some of these issues are hidden within the mkinitcpio-archiso scripts.