units: fix BindsTo= logic when applied relative to services with Type=oneshot

Start jobs for Type=oneshot units are successful when the unit state
transition activating → inactive took place. In such a case all units
that BindsTo= on it previously would continue to run, even though the unit
they dependet on was actually already gone.
core: minor modernizations
util: try to be a bit more NFS compatible when checking whether an FS is writable

https://bugs.freedesktop.org/show_bug.cgi?id=81169
Update TODO