kernel-pfkey: Add option to install routes via internal interface

On FreeBSD, enabling this selects the correct source IP when sending
packets from the gateway itself.

conf/plugins/kernel-pfkey.opt

@@ -5,3 +5,13 @@ charon.plugins.kernel-pfkey.events_buffer_size = 0
 	Because events are received asynchronously installing e.g. lots of policies
 	may require a larger buffer than the default on certain platforms in order
 	to receive all messages.
+
+charon.plugins.kernel-pfkey.route_via_internal = no
+	Whether to use the internal or external interface in installed routes.
+
+	Whether to use the internal or external interface in installed routes.
+	The internal interface is the one where the IP address contained in the
+	local traffic selector is located, the external interface is the one over
+	which the destination address of the IPsec tunnel can be reached.
+	This is not relevant if virtual IPs are used, for which a TUN device is
+	created that's used in the routes.