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.
Luca Boccassi
ee7c04873e