Performance: checking group enrolment key always iterates through all groups

low hanging fruit:

check_group_entry in enrol.php has a loop on groups, currently it iterates all the groups all the time:

foreach ($groups as $group) {
if ( !empty($group->enrolmentkey) and (stripslashes($password) == $group->enrolmentkey) )

}

maybe someone can put a break in when there is a match.