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 https://github.com/void-linux/void-mklive/pull/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/