Use PostgreSQL 9.3's pg_isready to wait for connectivity

The postgresql module has a postStart section that waits for a database
to accept connections before continuing. However, this assumes various
properties about the database - specifically the database user
and (implicitly) the database name. This means that for old
installations, this command fails because there is no 'postgres' user,
and the service never starts.

While 7deff39 does create the 'postgres' user, a better solution is to
use `pg_isready`, who's sole purpose is to check if the database is
accepting connections. This has no dependency on users, so should be
more robust.

nixos/modules/services/databases/postgresql.nix

@@ -226,7 +226,7 @@ in
         # Wait for PostgreSQL to be ready to accept connections.
         postStart =
           ''
-            while ! su -s ${pkgs.stdenv.shell} postgres -c 'psql postgres -c ""' 2> /dev/null; do
+            while ! ${pkgs.postgresql93}/bin/pg_isready > /dev/null; do
                 if ! kill -0 "$MAINPID"; then exit 1; fi
                 sleep 0.1
             done