Fix pg_verify_checksums on Windows.

Fix pg_verify_checksums on Windows.

To verify the checksums, we open the file in text mode which doesn't work
on Windows as WIN32 treats Control-Z as EOF in files opened in text mode.
This leads to "short read of block .." error in some cases.

Fix it by opening the files in the binary mode.

Author: Amit Kapila
Reviewed-by: Magnus Hagander
Backpatch-through: 11
Discussion: https://postgr.es/m/CAA4eK1+LOnzod+h85FGmyjWzXKy-XV1FYwEyP-Tky2WpD5cxwA@mail.gmail.com