Multiple configuration support

NSsbackup should be able to handle multiple configuration:

Use case ( quoted Bug #141002 ) :

I wish that you could define multiple backups in sbackup, each with its own rules as how many backups to store, when to create the backups, etc.
The reasoning behind this is that I have a huge music collection, and an equally huge pictures directory. I want to backup both the music and the pictures, but if I tell sbackup to do it together with the system-files backup, my backup hd would be full in a few days.
So it would be ideal if I could have one backup rule for the system files (backup often, save lots of backups), and one for the music and images (backup once per day, but stay incremental for a long time and never have more than 1 full backups on hand at a time) - that would be great!

EndQuote

For this, we will need to define a standard naming of nssbackup configuration file ( maybe nssbackup[.*].conf where [.*] is a regexp ) and define a directory in which nssbackup will search for the config files .Currently nssbackup is using XDG specification for simple users . so the /home/user/.config/nssbackup dir is a good candidate ( for simple users )
For admins , we will have to create a */etc/nssbackup.d* directory that will contains all special configuration.