tree-wide: introduce LOG_ITEM() macro for verifying format string (#36780)

This introduce `LOG_ITEM()` macro that checks format strings in
log_struct() and friends.

Hopefully, this silences false-positive warnings by Coverity.

diff --cc src/core/cgroup.c
Simple merge

diff --cc src/core/exec-invoke.c
Simple merge

diff --cc src/journal/journald-server.c
Simple merge

diff --cc src/login/logind-action.c
Simple merge