cgroup-util: add cg_is_delegated_fd() helper

This is just like cg_is_delegate() but operates on an fd instead of a
cgroup path.

Sooner or later we should access cgroupfs mostly via fds rather than
paths, but we aren't there yet. But let's at least get started.

src/basic/cgroup-util.h

@@ -211,6 +211,7 @@ int cg_rmdir(const char *controller, const char *path);
 int cg_is_threaded(const char *path);
 
 int cg_is_delegated(const char *path);
+int cg_is_delegated_fd(int fd);
 
 int cg_has_coredump_receive(const char *path);