systemctl: add --kernel-cmdline-reuse option

kexec-tools has a --reuse-cmdline option which is very convenient
when doing a lot of reboots, add the same to systemctl.
Dedup options, letting the last one wins in case of duplicates,
so that 'systemctl kexec --reuse-cmdline' can be chained many times
without continuosly expanding the cmdline with duplicates from
the boot entry.

shell-completion/bash/systemctl.in

@@ -134,7 +134,7 @@ _systemctl () {
                       --help -h --no-ask-password --no-block --legend=no --no-pager --no-reload --no-wall --now
                       --quiet -q --system --user --version --runtime --recursive -r --firmware-setup
                       --show-types --plain --failed --value --fail --dry-run --wait --no-warn --with-dependencies
-                      --show-transaction -T --mkdir --read-only'
+                      --show-transaction -T --mkdir --read-only --kernel-cmdline-reuse'
         [ARG]='--host -H --kill-whom --property -p -P --signal -s --type -t --state --job-mode --root
                --preset-mode -n --lines -o --output -M --machine --message --timestamp --check-inhibitors --what
                --image --boot-loader-menu --boot-loader-entry --reboot-argument --kernel-cmdline --drop-in --when'

shell-completion/zsh/_systemctl.in

@@ -543,4 +543,5 @@ _arguments -s \
     '--failed[Show failed units]' \
     '--timestamp=[Change format of printed timestamps]:style:_systemctl_timestamp' \
     '--when=[Schedule halt/power-off/reboot/kexec action after a certain timestamp]:timestamp:(show cancel auto)' \
+    '--kernel-cmdline-reuse[Reuse the current kernel command line when loading the kexec kernel]' \
     '*::systemctl command:_systemctl_commands'