radvd: Install and configure radvd

The *radvd* role and `radvd.yml` playbook can be used to configure the
`radvd` IPv6 SLAAC router advertisement daemon on Linux.

roles/radvd/templates/radvd.conf.j2

@@ -0,0 +1,28 @@
+{% for iface in radvd_interfaces %}
+interface {{ iface.interface }}
+{
+	AdvSendAdvert on;
+{% if iface.other_config|d(False) %}
+	AdvOtherConfigFlag on;
+{% endif %}
+
+	prefix {{ iface.prefix }} {
+		AdvOnLink {% if iface.on_link|d(True) %}on{% else %}off{% endif %};
+		AdvAutonomous {% if iface.autonomous|d(True) %}on{% else %}off{% endif %};
+	};
+{% if iface.rdnss is defined %}
+{% for address in iface.rdnss %}
+
+	RDNSS {{ address }} {
+	};
+{% endfor %}
+{% endif %}
+{% if iface.dnssl is defined %}
+{% for domain in iface.dnssl %}
+
+	DNSSL {{ domain }} {
+	};
+{% endfor %}
+{% endif %}
+};
+{% endfor %}