Failed to fetch subvolume detail / readlink: unrecognized option: e

Hi,

I downloaded the master version of btrbk today to try to backup the content of one SDD onto another one on my Turris Omnia router+NAS running [Turris OS](https://www.turris.com/en/turris-os/). But I get the error message:

`Failed to fetch subvolume detail [...] readlink: unrecognized option: e `

I have never done BTRFS backups or used btrbk before... Am I doing something incorrect?

Here is some more information:

> root@turris:~# uname -a
> Linux turris 4.14.254 #0 SMP Tue Nov 23 09:52:21 2021 armv7l GNU/Linux
>
> root@turris:~# grep mnt /etc/mtab
> /dev/sda1 /mnt/backup btrfs rw,relatime,space_cache,subvolid=5,subvol=/ 0 0
> /dev/sdb1 /mnt/nas btrfs rw,relatime,space_cache,subvolid=5,subvol=/ 0 0
>
> root@turris:~# btrbk ls /
> MOUNT_SOURCE ID FLAGS PATH
> /dev/mmcblk0p1 256 - /
> /dev/sda1 5 - /mnt/backup
> /dev/sdb1 5 - /mnt/nas
>
> root@turris:~# cat /etc/btrbk.conf
> timestamp_format long
> snapshot_preserve_min 18h
> snapshot_preserve 48h
> snapshot_dir /mnt/backup
> subvolume /mnt/nas/florent
>
> root@turris:~# btrbk --dry-run run
> WARNING: Skipping subvolume "/mnt/nas/florent": Failed to fetch subvolume detail
> WARNING: ... Command execution failed (exitcode=1)
> WARNING: ... sh: readlink -v -e '/mnt/nas/florent'
> WARNING: ... readlink: unrecognized option: e
> WARNING: ... BusyBox v1.30.1 () multi-call binary.
> WARNING: ...
> WARNING: ... Usage: readlink [-fnv] FILE
> WARNING: ...
> WARNING: ... Display the value of a symlink
> WARNING: ...
> WARNING: ... -f Canonicalize by following all symlinks
> WARNING: ... -n Don't add newline
> WARNING: ... -v Verbose
> --------------------------------------------------------------------------------
> Backup Summary (btrbk command line client, version 0.32.0-dev)
>
> Date: Sun Dec 5 16:26:00 2021
> Config: /etc/btrbk.conf
> Dryrun: YES
>
> Legend:
> === up-to-date subvolume (source snapshot)
> +++ created subvolume (source snapshot)
> --- deleted subvolume
> *** received subvolume (non-incremental)
> >>> received subvolume (incremental)
> --------------------------------------------------------------------------------
> /mnt/nas/florent
> !!! Aborted: Failed to fetch subvolume detail
>
> NOTE: Some errors occurred, which may result in missing backups!
> Please check warning and error messages above.
>
> NOTE: Dryrun was active, none of the operations above were actually executed!

Start with the `btrbk --dry-run run` reproduction and the reported `sh: readlink -v -e '/mnt/nas/florent'` invocation; compare it with BusyBox v1.30.1's supported `readlink` options. Done means the subvolume is no longer skipped on this Turris OS environment and the dry run completes without the readlink error.

Written by the indexing model from the issue text.