checkbox() sanitizes HTML attribute values.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5156 2fea6a4d-a838-0410-917b-93a53c48e9d2
Clean left-over argument tail.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5157 2fea6a4d-a838-0410-917b-93a53c48e9d2