Use the allocation size for the file "desc" block.  This is important for direct I/O, so that reads and writes are aligned as expected.
__wt_block_ckpt_init() shouldn't set the checkpoint's file size to a
newly created file size, that should be set by the caller.  (There's
only a couple places that aren't cracking an existing checkpoint block
and would theoretically need to set it, and actually none of them need
to set it, it's really only useful when it's taken out of an existing,
written, checkpoint that we're subsequently reading.)

Review everywhere "sector" is used, and change it to something less
specific.
Merge pull request #533 from wiredtiger/desc-allocsize

Use the allocation size for the file "desc" block.