core: do not post-process skipped mounts

core: do not post-process skipped mounts

When a mount is gracefully skipped (e.g.: BindReadOnlyPaths=-/nonexistent)
we still post-process it, like making it read-only. Except if nothing
has been mounted, the mount point will be made read-only for no reason.
Track when mounts are skipped and avoid post-processing.

One day we'll switch all of this to the new mount api and do these
operations atomically or not at all.

Fixes https://github.com/systemd/systemd/issues/29725