logind: get rid of X11 display socket symlink

X11 never made use of it anyway and it's probably better to just push
$DISPLAY into the systemd daemon from gnome-session (or equivalent
program) than to change libX11 to look for this socket. In particular
since we won't need this for Wayland anyway and we shouldn't add
infrastructure for stuff that's on its way out anyway.
systemctl: introduce new "import-environment" command

This may be used in graphical session start-up scripts to upload
environment variables such as $DISPLAY into the systemd manager easily.
event: extend priority raneg to 64bit

The kdbus prioq logic will use 64bit prios too, so let's keep this
similar in style. Using 64bit here has the advantage, that pointers and
pretty much anything else can be mapped naturally to priorities if so
desired.