hotplug: reliably handle interfaces with ubus zone hints

hotplug: reliably handle interfaces with ubus zone hints

So far, the firewall hotplug did not initiate a reload for interfaces which
are not covered in the firewall configuration but provide a zone hint in
their ubus data section.

Extend the hotplug script to handle this case by checking whether a zone
hint is present and if the requested zone exists in the configuration if
a direct zone lookup fails.

Fixes: #9611
Signed-off-by: Jo-Philipp Wich <jo@mein.io>