Update backuppc plugin, fix issues with hostnames with dot(s) (.)

We encountered an issue where the hostnames in backuppc had also the domain/subdomain names. As dots are not allowed in munin datasource names we had to fix the backuppc script. Ours is as above, you can find the explanation for clear_fieldnames at : http://munin-monitoring.org/wiki/notes_on_datasource_names.