Add progress reporting to pg_verifybackup

Hi all,

I've attached the simple patch to add the progress reporting option to
pg_verifybackup. The progress information is displayed with --progress
option only during the checksum verification, which is the most time
consuming task. It cannot be used together with --quiet option.

Feedback is very welcome.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com