Add USER SET parameter values for pg_db_role_setting

The USER SET flag specifies that the variable should be set on behalf of an
ordinary role.  That lets ordinary roles set placeholder variables, which
permission requirements are not known yet.  Such a value wouldn't be used if
the variable finally appear to require superuser privileges.

The new flags are stored in the pg_db_role_setting.setuser array.  Catversion
is bumped.

This commit is inspired by the previous work by Steve Chavez.

Discussion: https://postgr.es/m/CAPpHfdsLd6E--epnGqXENqLP6dLwuNZrPMcNYb3wJ87WR7UBOQ%40mail.gmail.com
Author: Alexander Korotkov, Steve Chavez
Reviewed-by: Pavel Borisov, Steve Chavez

doc/src/sgml/ref/psql-ref.sgml

@@ -1897,6 +1897,13 @@ INSERT INTO tbl1 VALUES ($1, $2) \bind 'first value' 'second value' \g
         commands are used to define per-role and per-database configuration
         settings.
         </para>
+
+        <para>
+        Since <productname>PostgreSQL</productname> 16 the output includes
+        column with the values of
+        <link linkend="sql-alterrole-user-set"><literal>USER SET</literal></link>
+        flag for each setting.
+        </para>
         </listitem>
       </varlistentry>