More consistency checks when a ZGY file is opened.

Refuse to open a truncated ZGY file and instead produce a good error message. Truncation is a common problem when copying an existing file with generic OS commands and running out of space. Or uploading with sdutil and have the network connection drop. The problem is detected by looking for bricks that claim to be stored past EOF.

Also refuse to open a file where multiple bricks point to the same disk location. Arguably that might have been allowed for running extreme stress tests without needing petabytes of storage. But currently that trick will confuse the reader so the tests will fail for that reason.