Idea: Preseed mode for portability

Use case: I as a user store my Firefox AppImage in a read-only loction, but nevertheless want it to use my custom pre-configuration. I do _not_ want to write back changes to the profile to the read-only location (e.g., in order to not ever change my defaults, or collect persistent cookies).

We have portable mode which means that a custom `$HOME` is used. In some situations it may be desirable to only pre-seed `$HOME`, e.g., add initial configuration if it is not there yet, along those lines:

```
if [ ! -e "$HOME/.mozilla" ] ; then
cp -r /isodevice/Applications/Firefox-64.0.glibc2.3.4-x86_64.AppImage.home/.mozilla "$HOME"
sudo chown -R "$USER" "$HOME/.mozilla"
fi
```

How can we do this efficiently in `runtime.c`? Especially the permission setting should be done directly when copying since we cannot assume we can use sudo.