Bug#969247: FIX: add /etc/boinc-client/config.properties

[Marcel Partap]

.. so after some research, this happens due to these changes merged in March: https://github.com/BOINC/boinc/pull/3709

Fortunately, previous behaviour can easily be restored by adding a /etc/boinc-client/config.properties file containing:

> data_dir=/var/lib/boinc

That points boincmgr and boinccmd where to find the gui_rpc_auth.cfg file to read the password from.
In order for a random password (in case of an empty file) to be written successfully, I think I had to change the permission for either the sym link /var/lib/boinc-client/gui_rpc_auth.cfg or the actual file in /etc.

Regards!