OpenVPN: track interface disconnect

Setup disconnection callback and use it to properly track management
interface availability.

Change-Id: I11b8bc71c1e9cbbf992ee3b7c1c9aae6e6f697e0
Requires: libosmo-netif I905adb2d6191216551a3bcdcd1aec1f96f01612a
Related: SYS#2655
diff --git a/src/client.h b/src/client.h
index d878450..a14019e 100644
--- a/src/client.h
+++ b/src/client.h
@@ -15,7 +15,7 @@
 	/* name of this client */
 	const char *name;
 	/* remote host/IP */
-	const char *remote_host;
+	char *remote_host;
 	/* remote port */
 	uint16_t remote_port;
 };