[ENH] registration: Add a "secret" key pref to restrict access to tiki-register php.
Visitors (i.e. bots) arriving on tiki-register.php without this key parameter are sent back a 404 not found error and redirected to the home page.

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@51154 b456876b-0849-0410-b77d-98878d47e9d5