Adjust list of hardcoded excluded directories

Adjust list of hardcoded excluded directories

Only hardcode directories that never(!) make sense to backup, and introduce
new configuration variables "exclude_args_add" (for system configurations)
and "default_exclude_args_add" (for backup-script.conf) to set apropriate
settings and defaults for the local installation.

Now the following directories are always excluded:

/dev, /proc/, /sys/, /tmp, /var/tmp, /media, /mnt, /net, /var/cache/apt,
and /var/log.