Fix 'writable' flag on mount

The writable flag is incorrect on mount.  This causes the snapshot to
be mounted read/write during the gosure, which ends up updating the
atimes (making the snapshots larger).  Future code, such as snapshots,
will rely on this being correct, however.
Implement lvm/ext4 mirrors

The 'push' command uses rsync and lvm2 thin snapshots on a destination
volume to mirror the lvm2 thin snapshots used on the source volume.
push: Run lvm snapshot after umounting

This will be slightly cleaner, doing the snapshot after the volumes
are unmounted.  Making snapshots of mounted volumes is safe in Linux,
so this really just makes things a little cleaner.