Support for setting/getting IPv6 TCLASS on socket level

[dvolodin7]

tclass is the IPv6 counterpart to IPv4's tos field.

We can implement IPv6 methods just like Socket::tos and Socket::set_tos

    pub fn set_tclass(&self, tclass: u32) -> io::Result<()> {
        unsafe { setsockopt(self.as_raw(), sys::IPPROTO_IPV6, sys::IPV6_TCLASS, tclass as c_int) }
    }
    pub fn tclass(&self) -> io::Result<u32> {
        unsafe {
            getsockopt::<c_int>(self.as_raw(), sys::IPPROTO_IPV6, sys::IPV6_TCLASS).map(|tclass| tclass as u32)
        }
    }

IPV6_CLASS option is defined in RFC3542.

[Thomasdezeeuw]

Same as #418 (comment), prs are welcome 👍