03-filesystems.sh ignores ro option in /etc/fstab for root fs

Even with `default,ro` in `/etc/fstab` for root fs (`/`) the filesystem is always mounted as read-write. Relevant part of `/etc/runit/core-services/03-filesystems.sh`:
```
msg "Mounting rootfs read-write..."
mount -o remount,rw / || emergency_shell
```