vrrp: Fix a memory leak while dropping incoming IPSEC-AH authenticated advert.

Jay True fixed a memory leak in IPSEC-AH sanity check. Digest was allocated in
previous code without freing it on HMAC-MD5 missmatch. good catch Jay !
vrrp: Extend IPSEC-AH auth to support unicast

If you plane to use IPSEC-AH auth in unicast mode (which THE best idea), then
IP header TTL MUST be zeroed since it is mutable field on transit.
vrrp: Update VRRP VMAC doc

Add vmac_xmit_base in configuration example and force rp_filter=0 on
macvlan interface.
vrrp: make gratuitous ARP repeat count configurable

Patrick Schaaf and I introduced 2 new keywords :

* garp_master_repeat : Gratuitous ARP count sent on the wire
                       after MASTER state transition.

* garp_master_refresh_repeat : Gratuitous ARP count sent on the
                               wire when garp_refresh_timer fired.