feat(vpn): auto detection of IPv6 support

- `OPENVPN_IPV6` removed
- Affects OpenVPN
- Use the same mechanism for OpenVPN and Wireguard
- Check only once at program start since this is unlikely to change at runtime
- Log if IPv6 is supported
- Remove `IPv6` boolean from settings structs
- Move IPv6 detection as a method on NetLinker

internal/netlink/ipv6.go

@@ -0,0 +1,30 @@
+package netlink
+
+import (
+	"fmt"
+
+	"github.com/vishvananda/netlink"
+)
+
+func (n *NetLink) IsIPv6Supported() (supported bool, err error) {
+	links, err := n.LinkList()
+	if err != nil {
+		return false, fmt.Errorf("listing links: %w", err)
+	}
+
+	for _, link := range links {
+		routes, err := n.RouteList(link, netlink.FAMILY_V6)
+		if err != nil {
+			return false, fmt.Errorf("listing IPv6 routes for link %s: %w",
+				link.Attrs().Name, err)
+		}
+
+		if len(routes) == 0 {
+			continue
+		}
+
+		return true, nil
+	}
+
+	return false, nil
+}