Remove all traces of g_manage_cookie_val by using gpc_get_cookie()

Also fix up manage_user_page so it actually uses the cookie again (it was using an isset() check instead of a straight boolean comparison of $f_save)

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2014 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9