systemd: add "requiredBy" option.

"require" is a stronger version of "want",
and just like wantedBy allows you to specify this relation in reverse,
requiredBy does the same.

It may seem pointless to be able to specify these stronger relations in
reverse, because if something is really required, you would expect the
other unit to specify this himself.

However, this is still useful for virtual/automatic units (like
devices) that are created by systemd on demand and hence have no unit
file you can alter.
virtual network interfaces: make systemd aware of the relation between device and service

If I want to bring down tap0.service (or systemd wants to do this
during a configuration-change which changes the path to tunctl),
openvpn (or other services using tap0) need to be brought down as
well, otherwise tunctl -d is not able to remove the tap0 device,
leaving it in a failed (but "up") state.
cron: set TZDIR so cron is able to find the TZ
Freenet: fix 2 typos
Don't set $TZ

We don't need to set $TZ, because we have /etc/localtime.  In fact,
setting $TZ without $TZDIR doesn't work anymore since Glibc no longer
contains zone info.
Create a /etc/zoneinfo symlink and use it in $TZDIR

Setting $TZDIR to ${pkgs.tzdata}/share/zoneinfo can cause logged-in
sessions to refer to a garbage-collected zoneinfo directory.  So use
/etc/zoneinfo instead.
lightdm: Initial expression for lightdm as the display-manager service (using default GTK greeter)
lightdm: improving a bit the indentation.
Putting the gnunet module up to date. It still doesn't start gnunet though.

No idea why.
Making gnunet start properly (calling gnunet-service-arm directly)