mldonkey: Join mldonkey user into freedombox-share group

Currently, if mldonkey has to operate on a folder the folder needs to be owned by 'mldonkey' user or the folder needs to have liberal permissions set on it. When an external disk is attached for NAS use case, we expect multiple application to be able to access this folder. Transmission and Deluge are already part of the freedombox-share group and can use folders that are shared by Samba. Thus all three of them can work on the same folder.

Make mldonkey user a part of freedombox-share group just like in case of Transmission and Deluge to fix this problem.

Test that folders created by Samba sharing can be written to by mldonkey.