vrrp: fix/extend gratuitous ARP handling

multiple people reported issues where MASTER didnt recover properly
after outage due to no gratuitous ARP sent. VRRP is a protocol designed
to be used between node plugged on the same layer2 in order to guarantee
link failure is directly linked to a protocol FSM handling (FAULT
transition). With current virtualization env quite every think can be
virtualized from host (VM) to network (vswitch). In some cases those
virtualized env offer a virtualized layer2 on which VRRP is plugged and
sometime forwarding or routing over this virtual path can be broken.

I extended gratuitous ARP handling in 2 ways :

(1) When a MASTER receive a higher prio advert it sends a last
    advert before transiting to BACKUP state. The immediate effect
    at remote MASTER side is to sollicite a gratuitous ARP
    broadcast.

(2) Add an optional support to periodic gratuitous ARP sending
    while in MASTER state. By default it is disabled but one
    can activate this feature by configuring keyword
    "garp_master_refresh" in seconds in vrrp_instance block
    (refer to keepalived.conf.SYNOPSIS).