fix #4477: init: symlink busybox binaries on init

fix #4477: init: symlink busybox binaries on init

This change makes Busybox symbolically link its inbuilt utils to their
respective locations, allowing them to be invoked through their
absolute path. This means that Busybox inbuilts like e.g. `mount`
and `env` become available at `/bin/mount` and `/usr/bin/env` etc.

In particular, when entering any of the snapshot directories in
`.zfs/snapshot` (e.g. via `cd` or `ls`), ZFS normally mounts the
respective snapshot(s) on demand, using `/usr/bin/env mount [...]`
internally. Before this change, this would fail, rendering ZFS
snapshots unavailable when restoring individual files.

Signed-off-by: Max Carrara <m.carrara@proxmox.com>