systemd-coredump: Add conflict with shutdown.target
    
    Otherwise a coredump started at the inconvinient moment can stop
    shutdown.target leaving the system in a halfway-down state:
    
    Pulling in shutdown.target/start from systemd-poweroff.service/start
    Added job shutdown.target/start to transaction.
    ...
    Keeping job shutdown.target/start because of systemd-poweroff.service/start
    ...
    [  OK  ] Stopped target Remote File Systems.
    shutdown.target: starting held back, waiting for: systemd-networkd.socket
    sysinit.target: stopping held back, waiting for: remount_tmp.service
    systemd-coredump.socket: Incoming traffic
    ...
    systemd-coredump@0-243-0.service: Trying to enqueue job systemd-coredump@0-243-0.service/start/replace
    Added job systemd-coredump@0-243-0.service/start to transaction.
    Pulling in systemd-journald.socket/start from systemd-coredump@0-243-0.service/start
    Added job systemd-journald.socket/start to transaction.
    Pulling in system.slice/start from systemd-journald.socket/start
    Added job system.slice/start to transaction.
    Pulling in -.slice/start from system.slice/start
    Added job -.slice/start to transaction.
    Pulling in system-systemd\x2dcoredump.slice/start from systemd-coredump@0-243-0.service/start
    Added job system-systemd\x2dcoredump.slice/start to transaction.
    Pulling in system.slice/start from system-systemd\x2dcoredump.slice/start
    Pulling in shutdown.target/stop from system-systemd\x2dcoredump.slice/start
    Added job shutdown.target/stop to transaction.
    ...
    Keeping job systemd-poweroff.service/stop because of umount.target/stop
    Keeping job shutdown.target/stop because of systemd-coredump@0-243-0.service/start
    
    (cherry picked from commit 4e947bd04944e58df4103eee4cb8180b5008f143)