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 4e947bd0)
(cherry picked from commit e11d3ec1)