feat: add VRRP unicast peer support for macvlan and restricted network environments

- Add unicast_src_ip and unicast_peers config fields
- NewUnicastSocket binds to source IP for proper packet reception
- SendTo() sends VRRP advertisements directly to peer IPs
- Receive loop filters packets from non-peer sources in unicast mode
- Backward compatible: multicast mode used when unicast_peers is empty
- Add unicast config example (etc/config.unicast.yaml)
- Update README with unicast documentation and examples

etc/config.unicast.yaml

@@ -0,0 +1,31 @@
+global:
+  router_id: "node1"
+
+vrrp_instances:
+  # Example: VRRP with unicast peers for macvlan environments
+  # In macvlan networks, multicast between sub-interfaces of the same
+  # parent interface does not work. Use unicast to send VRRP advertisements
+  # directly to peer IP addresses.
+  - name: "VI_1"
+    interface: "eth0"
+    state: "BACKUP"
+    virtual_router_id: 51
+    priority: 100
+    advert_interval: 1
+    auth_type: "PASS"
+    auth_pass: "secret123"
+    virtual_ips:
+      - "192.168.1.100/24"
+
+    # Unicast source IP: the local IP address to use for sending VRRP packets.
+    # If omitted, the interface's primary IPv4 address is used automatically.
+    unicast_src_ip: "192.168.1.10"
+
+    # Unicast peer list: IP addresses of the other VRRP nodes.
+    # When configured, VRRP advertisements are sent directly to these IPs
+    # instead of using multicast (224.0.0.18).
+    # On each node, list the OTHER nodes' IPs here (not your own).
+    unicast_peers:
+      - "192.168.1.11"
+      # Add more peers for setups with more than two nodes:
+      # - "192.168.1.12"