in_systemd: Process enumerated data as cfl_kvlist(s) at first

This is because systemctl's -o json-pretty or -o json converts
duplicated keys' values as array(s).
To avoid generating the duplicated key(s) does not resolve this issue.
Instead, we need to store as cfl_kvlist at first to detect duplicated
keys on enumerated data in journal storage.
Then, we also need to generate as msgpack's array format when the
duplicated key(s) were detected and translated as array format when
storing as cfl_kvlist(s).

Signed-off-by: Hiroshi Hatake <[email protected]>