vrrpd: use if_is_operative()

vrrpd: use if_is_operative()

Checks for interface usability instead of admin state, which is what I
wanted anyway. Also removes the operstate check when binding interfaces.
This way we can bind currently inoperative interfaces, won't start until
they're at least admin up, but *will* start if they're carrier down,
because we can fix that (and probably caused it :)

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>