Improve disk space alerts
    
    In an attempt to improve alerting accuracy and to be more lenient
    with "low" disk space on machines with large partitions:
    
    - keep current 90% critical alert for partitions with a total size
      of less than 94G
    - add a 1% critical alert for partitions with a total size of more
      than 94G
    - adjust future prediction to warn 12h in advance if 6h worth of
      data suggest free space might subceed zero
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>