udisks2: add udisks2 service

Udisks2 is a Dbus service started by systemd on demand.
Xfce: use udisks2 instead of udisks

Enabling udisks2 allows gvfs (which should be built with udisks support) to
perform mount/unmount operations on removable drives. It affects Thunar and
probably other Gtk-based file managers.

Gvfs doesn't supports earlier versions of udisks.
rabbitmq.nix: Set SYS_PREFIX empty so files live in /var

Signed-off-by: Shea Levy <shea@shealevy.com>
Merge pull request #132 from ierton/udisks2-v2

add udisks2 for Xfce
Adding freenet module
Static nameservers have precedence
Merge branch 'master' of https://github.com/NixOS/nixos into nameservers

Conflicts:
	modules/tasks/network-interfaces.nix
Merge pull request #147 from offlinehacker/nameservers

Static nameservers should have precedence
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