Add alternative hostname and service name helpers

[devgianlu]

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[devgianlu]

@alexpevzner Any chance for this to get merged? These helpers are quite important for implementing service name conflict resolution.

[alexpevzner]

Any chance for this to get merged?

Yes — sorry for not spotting this earlier.

To get this merged, we'll need proper function comments that explain what each function does, not just what it calls.

One of the core values of this project is providing clear, usable documentation for Avahi's API, since the native documentation is often opaque and incomplete. So simply stating that AlternativeHostname calls avahi_alternative_host_name won't be enough — we need to explain the semantics: what the function actually does, why you'd use it, and what the expected behavior is.

Could you add that? Happy to review once it's in.