So far we had MountFlags = "private", but as @Infinisil has correctly
noticed, there is a dedicated PrivateMounts option, which does exactly
that and is better integrated than providing raw mount flags.
When checking for the reason why I used MountFlags instead of
PrivateMounts, I found that at the time I wrote the initial version of
this module (Mar 12 06:15:58 2018 +0100) the PrivateMounts option didn't
exist yet and has been added to systemd in Jun 13 08:20:18 2018 +0200.
Signed-off-by: aszlig <aszlig@nix.build>
52299bccf5