in backup state notify when vrrp is not up and move to FAULT state
check if interface is known when using use_vmac

vrrp->ifp is NULL when use_vmac keyword is defined before the
interface keyword. This would result in a segfault
(http://sourceforge.net/p/keepalived/mailman/message/33083027/).
Merge pull request #105 from frankbb/master

in backup state notify when vrrp is not up and move to FAULT state