* Move DB code for removing a session in logout.php to a new function in
acctfuncs.inc.php
* Add ability for clear_expired_sessions function to check for DB connection
* Centralization of DB code important in a future transition to PDO interface

Signed-off-by: canyonknight <canyonknight@gmail.com>
Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>