openvpn: fixed to disable pam auth plugin if pam is not installed effectively

Note:

Starting with openvpn 2.3.0 there seem to be changes how plugins are enabled
and there are configure switches now for disabling the shipped plugins.

network/openvpn/openvpn.conf

@@ -53,7 +53,9 @@ if ! pkginstalled lzo ; then
 fi
 
 # pam authentication plugin
-pkginstalled -f pam && var_append plugins_enabled ' ' "auth-pam"
+if ! pkginstalled -f pam; then
+	var_append extraconfopt ' ' "--disable-plugin-auth-pam"
+fi
 
 # build plugins when $plugins_enabled is not empty
 if [ ! -z "$plugins_enabled" ]; then