openvpn: handler: refine netifd routing and config

openvpn: handler: refine netifd routing and config

Introduce a new `ipv6` proto option for OpenVPN netifd integration and
export it to the hotplug environment. IPv6 remains enabled by default,
but can now be explicitly disabled per instance.

Update the hotplug helper to apply IPv6 addresses and routes only when
IPv6 is enabled, allowing cleaner IPv4-only tunnel deployments.

Also improve route handling by:
- ignoring invalid default gateway values (0.0.0.0 / ::)
- replacing fixed `seq` loops with shell-safe while loops
- keeping trusted peer host routes conditional on valid gateways

Signed-off-by: Chen Minqiang <redacted>