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? :)