Fix pg_extension_config_dump() to handle update cases more sanely.

If pg_extension_config_dump() is executed again for a table already listed
in the extension's extconfig, the code was blindly making a new array entry.
This does not seem useful.  Fix it to replace the existing array entry
instead, so that it's possible for extension update scripts to alter the
filter conditions for configuration tables.

In addition, teach ALTER EXTENSION DROP TABLE to check for an extconfig
entry for the target table, and remove it if present.  This is not a 100%
solution because it's allowed for an extension update script to just
summarily DROP a member table, and that code path doesn't go through
ExecAlterExtensionContentsStmt.  We could probably make that case clean
things up if we had to, but it would involve sticking a very ugly wart
somewhere in the guts of dependency.c.  Since on the whole it seems quite
unlikely that extension updates would want to remove pre-existing
configuration tables, making the case possible with an explicit command
seems sufficient.

Per bug #7756 from Regina Obe.  Back-patch to 9.1 where extensions were
introduced.

doc/src/sgml/extend.sgml

@@ -665,6 +665,10 @@ SET LOCAL search_path TO @extschema@;
      and reload.
     </para>
 
+   <indexterm>
+    <primary>pg_extension_config_dump</primary>
+   </indexterm>
+
     <para>
      To solve this problem, an extension's script file can mark a table
      it has created as a configuration table, which will cause
@@ -703,6 +707,14 @@ SELECT pg_catalog.pg_extension_config_dump('my_config', 'WHERE NOT standard_entr
      be modified by users, can be handled by creating triggers on the
      configuration table to ensure that modified rows are marked correctly.
     </para>
+
+    <para>
+     You can alter the filter condition associated with a configuration table
+     by calling <function>pg_extension_config_dump</> again.  (This would
+     typically be useful in an extension update script.)  The only way to mark
+     a table as no longer a configuration table is to dissociate it from the
+     extension with <command>ALTER EXTENSION ... DROP TABLE</>.
+    </para>
    </sect2>
 
    <sect2>