Partial fix for #4389.
* Fix invalid username searches returning all posts instead of no posts.
* Fix "user:A user:B" returning results for user:B instead of no results.
* Fix "approver:A approver:B" returning results for approver:B instead of no results.
* Add support for negated -commenter, -noter, -noteupdater, -upvote, -downvote metatags.
* Add support for "any" and "none" values for all username metatags,
including negated metatags that didn't support "any" or "none" before.
* Change noter:any and commenter:any to include posts with deleted notes
or comments. Note that commenter:<username> already included deleted
comments before. This is so that commenter:any has the same behavior
as commenter:<username>