bad use of recordset return value for checking if empty.

with the changes in 2.0 the following code won't work correctly as recordset calls will always return some data and aren't empty.

here are just a couple of recordset_sql calls I've found but there are a lot more that need fixing.

this one looks really bad in lib/dml/moodle_database.php:
if ($mrs = $this->get_recordset_sql($sql, $params, 0, 1))

admin/multilangupgrade.php:
if ($rs = $DB->get_recordset_sql($sql)) {

course/user.php:
if (!$rs = $DB->get_recordset_sql($sql)) {

enrol/authorize/localfuncs.php:
if (!$rs = $DB->get_recordset_sql($sql)) {

group/overview.php:
if ($rs = $DB->get_recordset_sql($sql, $params)) {

lots like this in lib/accesslib.php:
if ($rs = $DB->get_recordset_sql($sql, $params)) {