core: add cg_path_get_unit_path()

From a given cgroup path, cg_path_get_unit() allows to retrieve the
unit's name. Although, this removes the path to the unit's cgroup,
preventing the result to be used to fetch xattrs.

Introduce cg_path_get_unit_path() which provides the path to the unit's
cgroup. This function behave similarly to cg_path_get_unit() (checking
the validity and escaping the unit's name).

src/basic/cgroup-util.h

@@ -261,6 +261,7 @@ int cg_path_get_cgroupid(const char *path, uint64_t *ret);
 int cg_path_get_session(const char *path, char **session);
 int cg_path_get_owner_uid(const char *path, uid_t *uid);
 int cg_path_get_unit(const char *path, char **unit);
+int cg_path_get_unit_path(const char *path, char **unit);
 int cg_path_get_user_unit(const char *path, char **unit);
 int cg_path_get_machine_name(const char *path, char **machine);
 int cg_path_get_slice(const char *path, char **slice);