Some code cleanup; better feedback on register

 * Made the errors show up on the register screen
 * Changed a <? to <?php, which was necessary for it to work in my setup of PHP (no short tags)
 * Require user to be logged in to view userpage.php (i.e. don't allow them to navigate there directly)
 * Added some conditionals to check for existence of variables to avoid the "undefined variable" warnings
 * Added a call to date_default_timezone_set to eliminate another PHP warning (defaults to UTC, can be overriden)