shell-completion: remove load from systemctl
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.
sd-dhcp-client: fix memory corruption
sd-rtnl: only alter link flags when reqeusted to
sd-dhcp-client/networkd: add interface MTU support
build-sys: fix --enable-dbus

The incorrect shell code of AS_IF always get false value, so it always
build without dbus though build with "--enable-dbus" explicitely.
bus: break reference cycle between bus and messages

Previously (6ee4f99 bus: break reference cycle between bus and
messages) I committed the test code, but not the actual fix :)
build-sys: increase valgrind stack limit so all tests pass again
bus: fix issue with reference counting

b7fc42e03 introduced a regression.