Add ip_mreqn for Fuchsia

Adds the ip_mreqn struct for the Fuchsia platform, as defined by ip(7).
Enables joining an IPv4 multicast group by NIC ID rather than by its
assigned IPv4 address.

Author: puradox

src/fuchsia/mod.rs

@@ -209,6 +209,12 @@ s! {
         pub imr_interface: in_addr,
     }
 
+    pub struct ip_mreqn {
+        pub imr_multiaddr: in_addr,
+        pub imr_address: in_addr,
+        pub imr_ifindex: ::c_int,
+    }
+
     pub struct ipv6_mreq {
         pub ipv6mr_multiaddr: in6_addr,
         pub ipv6mr_interface: ::c_uint,