Attempt to make QFile I/O 64-bit safe

Use qint64 wherever possible. The linear buffer is never requested to
allocate that much memory (always limited), but at least we ensure we're
not dropping bits where we shouldn't.

Windows's POSIX compatibility layer is never largefile enabled, so it is
always necessary to chunk large reads and writes. On Unix, this will
be rare, unless someone passed -no-largefile to configure, for some
weird reason.

Unfortunately, this is not testable, unless we can allocate a buffer
with 4 GB or more in size. The test for this would be to open a file we
know to be small, then try to read 4 GB + 1 byte. If everything works
correctly, we'll read the full file; if there was a truncation, we'd
read one byte.

Change-Id: If3ee511bf1de17e0123c85bbcaa463b9972746ce
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>