Finish the zstd compression implementation in liveslak

The new zstd compression support for squashfs modules allows for a much
snappier system because zstd decompression is 5 times faster than that of
xz (see https://sourceforge.net/p/squashfs/mailman/message/35989805/), but
at the expense of ISO sizes that are at least 10% bigger. The latter means
that we have to do some more pruning of the XFCE and PLASMA5 images to make
these fit in a CDROM and DVD image, respectively.

Here are some initial benchmarks with PLASMA5 Live ISO images,
booting in QEMU (times are in minutes:seconds):

Stage                         ZSTD  XZ
---------------------------------------
Boot to runlevel 4 start     0:39  0:50
SDDM Login Manager visible   0:55  1:26
Plasma5 desktop ready        1:42  3:17

I also tested two of the bigger applications, measuring the time between
entering the command in a terminal and having a usable application window:

Stage                         ZSTD  XZ
----------------------------------------
LO Writer window visible      0:17  0:34
Chromium window visible       0:09  0:14

It is obvious that a big Desktop Environment like Plasma5 where a lot of
binaries have to be loaded from their squashfs modules benefits a lot
from zstd, because after booting, the Plasma5 DE is available in
roughly 53% of the time it takes when using xz compression.

Zstd support in squashfs was added to Linux kernel 4.14. That means,
no customimzation of Slackware is required to make the Live OS work from
zstd-compressed squashfs modules.
In order to *create* these zstd-compressed squashfs modules, you will need
some custom packages at the moment, until they get added to Slackware-current:
zstd and a rebuild squashfs-tools to add the lacking zstd support to
mksqhashfs/unsquashfs.

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