Copy scope id when binding to IPv6 Link-Local address

[ThymoNL]

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Configuring a listener with bind_interface on an interface with an ipv6 link-local address causes Mosquitto to exit with "Error: invalid argument". This happens due to the the IPv6 scope ID not being copied to the returned addrinfo struct when resolving interfaces matching bind_interface.

This PR adds a commit that also copies the sin6_scope_id when a matching interface is found.

Fixes #2696

[ThymoNL]

@ralight May I gently bump this your way? Hoping you can take a look at this bugfix. 🙂

[ThymoNL]

@ralight Can I do anything else to answer your questions in order to get this merged? :)

[ThymoNL]

@ralight friendly reminder 🙂

[ralight]

@ThymoNL Thanks for the reminder. I would be happy to merge this, however I'd be much happier if there was a test to check the link local case. Any ideas on adding one?

[ralight]

Having looked into this a bit more, I think it's going to be impractical to write a general test for it.

The github runners do get a link local address:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 60:45:bd:4a:dd:40 brd ff:ff:ff:ff:ff:ff
    inet 10.1.0.23/16 metric 100 brd 10.1.255.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::6245:bdff:fe4a:dd40/64 scope link 
       valid_lft forever preferred_lft forever

However in other places, like my home, I get other non-link local IPv6 addresses and the bug does not manifest there in the same way.

[ThymoNL]

Yeah, I remember looking at this back then and not finding a trivial way to write a test either. Thanks for merging!