device: create units with intended "found" value

Change device_found_node() to also create a .device unit if a device is not
known by udev; this is the case for "tentative" devices picked up by mountinfo
(DEVICE_FOUND_MOUNT).  With that we can record the "found" attribute on the
unit.

Change device_setup_unit() to also accept a NULL udev_device, and don't
add the extra udev information in that case.

Previously device_found_node() would not create a .device unit, and
unit_add_node_link() would then create a "dead" stub one via
manager_load_unit(), so we lost the "found" attribute and unmounted everything
from that device.

https://launchpad.net/bugs/1444402
http://lists.freedesktop.org/archives/systemd-devel/2015-May/031658.html