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