Fixed db_exists function

As Frankenstein stated in forums the name has been changed
Added by default the DB prefix so the users don't have to bother typing
that(should we not include the prefix and allow users to check for
tables that do not belong to current Fusion installation?)
Moved the function in sqlhandler.inc.php, there is no need for a PDO
variant of this function as the methods used are PDO and MySQL
compatible