Varlink proxy for the io.systemd.Unit interface on PID 1’s socket.
Adapted from the interface definition in systemd’s
src/core/varlink-unit.c. The List method exists from v258, but the
per-type context sections monitord reads land later: src/core/varlink-service.c
and src/core/varlink-timer.c first appear in v261, so that is the real
minimum for anything here.