De-dupe XML namespaces in RSS automatically

When you are adding namespaces to the RSS feed via hook_nodeapi('rss item', ...) you don't want to worry about whether the namespace has been added before. This patch changes the namespace from a string to an associative array. The array keys should be the namespace prefix.

Nothing about hook_nodeapi('rss item', ...) is documented at the moment, so there is no documentation to go out of date.

I will provide a patch to the location module to use this.