doSlash(), txpspecialchars() and default values for setup and update

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5884 2fea6a4d-a838-0410-917b-93a53c48e9d2
Disallow arbitrary variables in plugin preview

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5885 2fea6a4d-a838-0410-917b-93a53c48e9d2