vrrpd: add initial macvlan support

vrrpd: add initial macvlan support

* Search for macvlan interfaces with the appropriate name and MAC
  address when starting up a new VRRP instance
* Split VRRP socket into two; one for Tx, one for Rx
* Bind Tx socket to the macvlan subinterface so our VRRP advertisements
  go out with the correct MAC address
* Send ARP requests from this macvlan subinterface
* Improve error messaging

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