DEBUG: Allow debug_fn to process __FILE__ and __LINE__
    
    In preparation for enabling journald support for the DEBUG logs, we
    will need to be able to pass in certain additional arguments that will
    be required, specifically the code file and line number.
    
    We will be able to optionally enable this in the file-based logs as well
    if we so choose, but for right now we will avoid breaking the log
    format on disk.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>