vchoi_fork

Prepare a bit to let the shadow index "float": drastically
simplify H5FD__vfd_swmr_index_deserialize(): reuse
h5_retry_init()/h5_retry_next() for retry loops.

Don't wait for the fstat(2) to read the correct size, because the
read(2) will return short if the file isn't long enough.  (This change
should save at least one system call, always.)

Leave a bunch of comments about the changes that I will have to make so
that the shadow index will float.

NFCI: do not cast H5MM_malloc() return values, this is not C++.