Breaking up hdf5file.c

[edhartnett]

In the fast open work (#857, also #489) I have found it convenient to break up the large hdf5file.c code file into:
hdf5fast.c - new code to do fast lazy var reads
hdf5slow.c - legacy code doing slow, non-lazy var reads
hdf5open.c - code involved in opening a file, used for both fast and slow opens
hdf5create.c - code involved in file creation
hdf5file.c - caching, enddef/redef, closing files (i.e. everything that's not opening or creating a file)

This division of the large hdf5file.c file makes sense even in the absence of lazy reads, and I propose to split up the file before presenting the lazy read work - otherwise it will be very hard to see just the lazy-read changes, since all the functions also get moved to new files.

So I will present a PR with minimal code changes to break the hdf5file.c code into:
hdf5open.c
hdf5create.c
hdf5file.c

Then, as a follow-on I can present the fast read code, which will mostly consist of adding hdf5slow.c and hdf5fast.c, plus some supporting changes in the other files. This will make it much easier to review.