nspawn: introduce --cleanup option to clear propagation and unix-export directories

This is useful when the previous invocation is unexpectedly killed.

Otherwise, if systemd-nspawn is killed forcibly, then unix-export
directory is not cleared and unmounted, and the subsequent invocation
will fail. E.g.
===
[   18.895515] TEST-13-NSPAWN.sh[645]: + machinectl start long-running
[   18.945703] systemd-nspawn[1387]: Mount point '/run/systemd/nspawn/unix-export/long-running' exists already, refusing.
[   18.949236] systemd[1]: systemd-nspawn@long-running.service: Failed with result 'exit-code'.
[   18.949743] systemd[1]: Failed to start systemd-nspawn@long-running.service.
===

shell-completion/bash/systemd-nspawn

@@ -66,7 +66,8 @@ _systemd_nspawn() {
 
     local -A OPTS=(
         [STANDALONE]='-h --help --version --private-network -b --boot --read-only -q --quiet --share-system
-                      --keep-unit -n --network-veth -j -x --ephemeral -a --as-pid2 -U --suppress-sync=yes'
+                      --keep-unit -n --network-veth -j -x --ephemeral -a --as-pid2 -U --suppress-sync=yes
+                      --cleanup'
         [ARG]='-D --directory -u --user --uuid --capability --drop-capability --link-journal --bind --bind-ro
                       -M --machine -S --slice -E --setenv -Z --selinux-context -L --selinux-apifs-context
                       --register --network-interface --network-bridge --personality -i --image --image-policy --tmpfs