unix: add MCAST_* on zos
unix: add MCAST_* on zos

Copied from golang.org/x/net/ipv6. This will allow to replace the
socket option constants in that package with constants from x/sys/unix.

Change-Id: Ie0ce8ac7a927418df6f6e793d626c425a1cf48ec
Reviewed-on: https://go-review.googlesource.com/c/sys/+/307670
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/sys,golang/sys,golang/sys