The CleanPool requires --more to be set and checks that in
`vl_method_clean_pool`. By switching to SD_VARLINK_REQUIRES_MORE
this will automatically be handled and is more clear to
the varlink users.
Based on the comment from Lennart in
https://github.com/systemd/systemd/pull/40650#discussion_r2832378002
and the work done by Mike in 09388a6b9e (thanks!).