Convert plugin output on signout page to new template system; plugins previously...

Convert plugin output on signout page to new template system; plugins previously hooking into the 'logout_above_text' hook should hook into the 'template_construct_signout.tpl' hook and return output in an array under a key called 'signout_message', such as: return array('signout_message' => )

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@11948 7612ce4b-ef26-0410-bec9-ea0150e637f0