Create the 'postgres' superuser

Old PostgreSQL installations were created using the 'root' database
user. In this case, we need to create a new 'postgres' account, as we
now assume that this is the superuser account.

Unfortunately, these machines will be left with a 'root' user as
well (which will have ownership of some databases). While PostgreSQL
does let you rename superuser accounts, you can only do that when you
are connected as a *different* database user. Thus we'd have to create a
special superuser account to do the renaming. As we default to using
ident authentication, we would have to create a system level user to do
this. This all feels rather complex, so I'm currently opting to keep the
'root' user on these old machines.

nixos/modules/services/databases/postgresql.nix

@@ -197,6 +197,7 @@ in
                 fi
                 rm -f ${cfg.dataDir}/*.conf
                 touch "${cfg.dataDir}/.first_startup"
+                touch "${cfg.dataDir}/postgresql-user-created"
             fi
 
             ln -sfn "${configFile}" "${cfg.dataDir}/postgresql.conf"
@@ -230,6 +231,11 @@ in
                 sleep 0.1
             done
 
+            if ! [ -e ${cfg.dataDir}/postgresql-user-created ]; then
+              createuser --superuser postgres
+              touch ${cfg.dataDir}/postgresql-user-created
+            fi
+
             if test -e "${cfg.dataDir}/.first_startup"; then
               ${optionalString (cfg.initialScript != null) ''
                 cat "${cfg.initialScript}" | su -s ${pkgs.stdenv.shell} postgres -c 'psql postgres'