Only require apcu, not apcu_bc

Fixes #27

design/privatefooter.php

@@ -24,7 +24,7 @@
 <?  if (!empty($LastActive)) { ?>
   <p>
     <a href="user.php?action=sessions">
-      <span class="tooltip" title="Manage sessions">Last activity: </span><?=time_diff($LastActive['LastUpdate'])?><span class="tooltip" title="Manage sessions"> from <?=(apc_exists('DBKEY')?(DBCrypt::decrypt($LastActive['IP'])):'[Encrypted]') ?></span>
+      <span class="tooltip" title="Manage sessions">Last activity: </span><?=time_diff($LastActive['LastUpdate'])?><span class="tooltip" title="Manage sessions"> from <?=(apcu_exists('DBKEY')?(DBCrypt::decrypt($LastActive['IP'])):'[Encrypted]') ?></span>
     </a>
   </p>
 <?  } ?>

design/privateheader.php

@@ -538,7 +538,7 @@ if (!empty($Alerts) || !empty($ModBar)) { ?>
           <?=implode(' | ', $ModBar); echo "\n"?>
         </div>
 <?  }
-if (check_perms('site_debug') && !apc_exists('DBKEY')) { ?>
+if (check_perms('site_debug') && !apcu_exists('DBKEY')) { ?>
         <div class="alertbar" style="color: white; background: #B53939;">
           Warning: <a href="tools.php?action=database_key">no DB key</a>
         </div>