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.

Built from https://develop.svn.wordpress.org/trunk@28743


git-svn-id: http://core.svn.wordpress.org/trunk@28557 1a063a9b-81f0-0310-95a4-ce76da25c4cd