Comment 1 for bug 1528822

I'd think the problem is somewhere in your PAM setup. For some reason "su" is asking for a password even when running as root:

    su -s /bin/sh postgres -c "test -O /var/lib/postgresql &&
            test -G /var/lib/postgresql" || \
        chown postgres:postgres /var/lib/postgresql

To be sure where it is really asking, add "set -x" to the beginning of /var/lib/dpkg/info/postgresql-common.postinst and do the dpkg --configure -a again.