File API cleanup

Here are a couple of patches that clean up the internal File API and
related things a bit:

0001-Update-types-in-File-API.patch

Make the argument types of the File API match stdio better:

- Change the data buffer to void *, from char *.
- Change FileWrite() data buffer to const on top of that.
- Change amounts to size_t, from int.

In passing, change the FilePrefetch() amount argument from int to
off_t, to match the underlying posix_fadvise().

0002-Remove-unnecessary-casts.patch

Some code carefully cast all data buffer arguments for
BufFileWrite() and BufFileRead() to void *, even though the
arguments are already void * (and AFAICT were never anything else).
Remove this unnecessary clutter.

(I had initially thought these casts were related to the first patch,
but as I said the BufFile API never used char * arguments, so this
turned out to be unrelated, but still weird.)