OPML:
* Learn how OPML works, cry.
* Remove unused globals in `startElement()`
* Stop using variable variables in `startElement()`

Tested with the OPML importer plugin

See #27881.


git-svn-id: https://develop.svn.wordpress.org/trunk@28743 602fd350-edb4-49c9-b593-d223f7449a82