report maximum-immutable-share-size correctly in light of filesystem limits

[zooko]

Currently storage servers report their maximum-immutable-share-size equal to their available disk space. This is not right for filesystems which limit filesize to a smaller size than the filesystem size, such as ext4, which limits files to 16 TB and filesystems to 1 EB.

​http://en.wikipedia.org/wiki/Comparison_of_filesystems#Limits

To fix this ticket, make the disk backend report the filesize limit separately from the available space (probably by hardcoding the facts from wikipedia's table into it) and make the storage server cap its reported maximum-immutable-share-size according to that limit.

The cloud storage backend has no hard limit on share size.