Fix overflow in Windows replacement pg_pread/pg_pwrite.

Fix overflow in Windows replacement pg_pread/pg_pwrite.

When calling the Windows file I/O APIs there is an implicit conversion
from size_t to DWORD, which could overflow.  Clamp the size at 1GB to
avoid that.

Not a really a live bug as we don't expect anything in PostgreSQL to
call with such large values.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/1672202.1703441340%40sss.pgh.pa.us