logind: Add a udev rule to tag all DRM cards with master-of-seat

logind: Add a udev rule to tag all DRM cards with master-of-seat

This is needed for generic DRM devices like the VirtualBox vboxvideo
driver, which exposes itself as a generic, ID-less DRM device at
/dev/dri/card0 (after applying this commit):
   $ udevadm info --query=all --path \
      /sys/devices/pci0000:00/0000:00:02.0/drm/card0
   P: /devices/pci0000:00/0000:00:02.0/drm/card0
   N: dri/card0
   E: DEVNAME=/dev/dri/card0
   E: DEVPATH=/devices/pci0000:00/0000:00:02.0/drm/card0
   E: DEVTYPE=drm_minor
   E: ID_FOR_SEAT=drm-pci-0000_00_02_0
   E: ID_PATH=pci-0000:00:02.0
   E: ID_PATH_TAG=pci-0000_00_02_0
   E: MAJOR=226
   E: MINOR=0
   E: SUBSYSTEM=drm
   E: TAGS=:master-of-seat:seat:uaccess:
   E: USEC_INITIALIZED=59893

Without this patch, the capabilities for a seat on a VirtualBox
installation of systemd v219 incorrectly show it as non-graphical, even
though I can type these commands from an xterm:
   $ loginctl show-seat seat0
   Id=seat0
   CanMultiSession=yes
   CanTTY=yes
   CanGraphical=no
   …

https://bugs.freedesktop.org/show_bug.cgi?id=90822