Merge pull request #29454 from poettering/cg-pidref-get-path

cgroup-util: add cg_pidref_get_path() helper and use it

diff --cc TODO
Simple merge

diff --cc src/basic/cgroup-util.c
Simple merge

diff --cc src/basic/cgroup-util.h
Simple merge

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