Set channel var to store IVR DTMF input

At present, IVR caller input is lost and not available later in the call flow. At present, you can input an Asterisk dial pattern as a valid IVR option, but since the DTMF input is lost, it has limited utility.

As proof of concept, I have made the following change to functions.inc.php and it seems to work just fine:

$ext->add($c, $e['selection'],'ivrsel-' . $e['selection'], new ext_setvar('IVR_RESULT', '${EXTEN}'));
$ext->add($c, $e['selection'],'ivrsel-' . $e['selection'], new ext_goto($e['dest']));