Spurious Backend Quota Warning

Incorrect Free Space Warnings likely fixes this, but please see if PR’s text fits:

The code before this would incorrectly always return the free space of / on non-Windows, instead of the actual mounted path’s free space.

Workaround is:

  --quota-disable (Boolean): Disable backend quota
    Disable the quota reported by the backend. The option --quota-size can
    still be used to set a manual quota
    * default value: false