Fix off_t overflow in pg_basebackup on Windows.

Fix off_t overflow in pg_basebackup on Windows.

walmethods.c used off_t to navigate around a pg_wal.tar file that could
exceed 2GB, which doesn't work on Windows and would fail with misleading
errors.  Use pgoff_t instead.

Back-patch to all supported branches.

Author: Davinder Singh <davinder.singh@enterprisedb.com>
Reported-by: Jakub Wartak <jakub.wartak@enterprisedb.com>
Discussion: https://postgr.es/m/CAKZiRmyM4YnokK6Oenw5JKwAQ3rhP0YTz2T-tiw5dAQjGRXE3Q%40mail.gmail.com