Auto merge of #2003 - puradox:patch-1, r=JohnTitor
Auto merge of #2003 - puradox:patch-1, r=JohnTitor

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.

rust-lang/libc,rust-lang/libc,rust-lang/libc