semplice-upgrade: added rules for numb and made the upgrade process more robust.

If there aren't any rule for the current codename (which may happen on
development releases), simply print out a warning message without raising
an Exception.
This avoids breaking the dist-upgrade process in these rare cases.
Also, ensure the program will iterate through every release with rules, so that
e.g. someone upgrading from Semplice 5 High Hopes still gets zram active.
semplice-upgrade: HeavenUpgrade: remove the systemd bits, as now systemd is installed by default as /sbin/init.

systemd-sysv will now be pulled automatically during the upgrade process, thus making the
/etc/default/grub editing process useless.