Use pseudofs functions already defined in lib.sh

[tsndqst]

The current mount_pseudofs() and umount_psuedofs() functions don't always allow umount to work properly.
The following errors are resolved by using the mount_pseudofs() and umount_psuedofs() functions from lib.sh.
The current functions are very similar to the ones in lib.sh so it seems like a useful change.
The lib.sh functions were updated in #245 to include --make-rslave which resolved these issues in the past for other scripts.

[10/11] Generating squashfs image (xz) from rootfs...
umount: /home/tsandqui/Code/void-mklive/mklive-build.lWvxd/image/rootfs/sys/fs/cgroup: target is busy.
ERROR: failed to unmount /home/tsandqui/Code/void-mklive/mklive-build.lWvxd/image/rootfs/sys/
umount: /home/tsandqui/Code/void-mklive/mklive-build.lWvxd/image/rootfs/sys/fs/cgroup: target is busy.
ERROR: failed to unmount /home/tsandqui/Code/void-mklive/mklive-build.lWvxd/image/rootfs/sys/

[classabbyamp]

iirc i tried this and it made things worse? maybe I was doing something weird.

[tsndqst]

That could be. I don't use mklive.sh very often. The other day I was cleaning up from a failed run and ended up deleting the host's /dev mount point. Luckily it worked find when I rebooted it.