Optimized and cleaned up list_multi_edit().

* Generates 2 queries instead of the maximum of 101.
* Moved the delete code to the switch statement.
* Fixed value validation; avoid type-juggling.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5315 2fea6a4d-a838-0410-917b-93a53c48e9d2