Fixing 4385 master

refs #4385 

Basically negative values are set to 0 - this is the identical behavior users can observe in the web ui as well.

In addition the storage information is path specific and a parameter to reflect this has been introduced.

This should fix various issues reported in the past regarding wrong used space in conjunction with external mounts

@danimo @karlitschek @kabum @icewind1991 THX