Update types in File API

Update types in File API

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().

Discussion: https://www.postgresql.org/message-id/flat/11dda853-bb5b-59ba-a746-e168b1ce4bdb%40enterprisedb.com