capabilities: Only plugins that require CAP_NET_ADMIN demand it

The daemon as such does not require this capability.

src/libcharon/plugins/kernel_libipsec/kernel_libipsec_plugin.c

@@ -102,6 +102,13 @@ plugin_t *kernel_libipsec_plugin_create()
 {
 	private_kernel_libipsec_plugin_t *this;
 
+	if (!lib->caps->keep(lib->caps, CAP_NET_ADMIN))
+	{	/* required to create TUN devices */
+		DBG1(DBG_KNL, "kernel-libipsec plugin requires CAP_NET_ADMIN "
+			 "capability");
+		return NULL;
+	}
+
 	INIT(this,
 		.public = {
 			.plugin = {