The native st_size-based approach to size estimation is broken because mutable
and immutable layouts have a different amount of overhead.  So start peeking
inside the share files.

Also, new_length must be respected now because when it is used the part of the
share file we peek at changes.  Also, new_length can *only* truncate so stop
treating it as if it can extend.

and without new_length

If enough valid passes are supplied for the amount of storage requested then
allow the operation to succeed.  Otherwise, fail with an error.

lots of mess to clean up though

It returns a list of Passes.  Encode them appropriately for Foolscap.