Add Ipv6MulticastHops as socket option

[sgasse]

What does this PR do

This PR adds the socket option Ipv6MulticastHops. With it, we can set the hop limit for all IPv6 multicast packets sent over a socket. It is equivalent to the following libc call:

let ret = unsafe {
    libc::setsockopt(
        socket.as_raw_fd(),
        libc::IPPROTO_IPV6,
        libc::IPV6_MULTICAST_HOPS,
        &[HOP_LIMIT] as *const _ as *const libc::c_void,
        std::mem::size_of::<libc::c_int>() as u32,
    )
};
Errno::result(ret)

If every packet shall have the same hop limit, this is an alternative to adding a control message.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API