'tahoe debug verify-share' command

[warner]

To assist with immutable-file repair, it would be useful to have a tool that you could point at a share file and it would tell you if all the internal hashes are valid. If you give it a URI too (a read- or verify- cap), then it should be able to compare the UEB hash against the one from the URI.

This should look like:

tahoe debug verify-share --uri URI:CHK:blah node*/storage/shares/ab/abcedSIetc/*
share 1: node1/storage/shares/ab/abSIetc/1 : all hashes good
share 4: node3/storage/shares/ab/abSTetc/4 : bad ciphertext tree hash
...