cgroups-agent: connect stdin/stdout/stderr to /dev/null
Inspired by https://github.com/systemd/systemd/pull/24024 this is
another user mode helper, where this might be an issue. hence let's
rather be safe than sorry, and also connect stdin/stdout/stderr
explicitly with /dev/null.
(cherry picked from commit 50492ce81589773df2d82b4fc8047778e86c6edf)