namespace: don't try to remount superblocks

We can't remount the underlying superblocks, if we are inside a user
namespace and running Linux <= 4.17.  We can only change the per-mount
flags (MS_REMOUNT | MS_BIND).

This type of mount() call can only change the per-mount flags, so we
don't have to worry about passing the right string options now.

Fixes #9914 ("Since 1beab8b was merged, systemd has been failing to start
systemd-resolved inside unprivileged containers" ... "Failed to re-mount
'/run/systemd/unit-root/dev' read-only: Operation not permitted").

> It's basically my fault :-). I pointed out we could remount read-only
> without MS_BIND when reviewing the PR that added TemporaryFilesystem=,
> and poettering suggested to change PrivateDevices= at the same time.
> I think it's safe to change back, and I don't expect anyone will notice
> a difference in behaviour.
>
> It just surprised me to realize that
> `TemporaryFilesystem=/tmp:size=10M,ro,nosuid` would not apply `ro` to the
> superblock (underlying filesystem), like mount -osize=10M,ro,nosuid does.
> Maybe a comment could note the kernel version (v4.18), that lets you
> remount without MS_BIND inside a user namespace.

This makes the code longer and I guess this function is still ugly, sorry.
One obstacle to cleaning it up is the interaction between
`PrivateDevices=yes` and `ReadOnlyPaths=/dev`.  I've added a test for the
existing behaviour, which I think is now the correct behaviour.

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