Fix storage of illegal_names. Add an upgrade routine to fix bad values.

Merges [24448] to the 3.5 branch. Bump DB version by one.

props SergeyBiryukov.
fixes #23418.



git-svn-id: https://core.svn.wordpress.org/branches/3.5@24449 1a063a9b-81f0-0310-95a4-ce76da25c4cd