pg_wrapper: If libreadline is installed, LD_PRELOAD this for "psql", to

avoid using the rather broken libedit. We need to build the postgresql-X.Y
packages against libedit for license reasons (#603599), but as libreadline
has a drop-in compatible ABI, this works around the licensing
restrictions. Thanks to Andreas Barth for working this out! Add a
recommends to libreadline6. (Closes: #608442, #607907, #607109, #611918)