Fixed the quota storage wrapper to correctly return the quota value when
the free space is not known (which usually happens when the disk_free_space
function is disabled)
This fixes the "used space" to be based on the unencrypted size, not
encrypted size, to be consistent with how quota/space is handled when
encryption is enabled
When calling fopen() on some storage types, these return false instead
of throwing an exception.
This fix makes sure that in case the stream wasn't opened (for example
when a file doesn't exist any more) the stream isn't wrapped.
Also added 'rb' as another case that doesn't need to be wrapped.
Fixes#6832
