ejabberd: allow one to change SPOOLDIR location

By default, ejabberd saves the mnesia database into /var/lib/ejabberd.
Otherwise, one might want to change this location (e.g., if /var/lib is
read-only). Add an option in the init script to offer this possibility.

Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>