unix: add SetsockoptPacketMreq on Linux

Add SetsockoptPacketMreq to set packet socket options used to
configure physical-layer multi‐casting and promiscuous mode
(PACKET_ADD_MEMBERSHIP, PACKET_DROP_MEMBERSHIP).

Change-Id: I0f292315ad1c57e6a3d1dd12f5f29be70d8bfd54
Reviewed-on: https://go-review.googlesource.com/c/sys/+/165977
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_linux.go

@@ -1022,6 +1022,10 @@ func SetsockoptIPMreqn(fd, level, opt int, mreq *IPMreqn) (err error) {
 	return setsockopt(fd, level, opt, unsafe.Pointer(mreq), unsafe.Sizeof(*mreq))
 }
 
+func SetsockoptPacketMreq(fd, level, opt int, mreq *PacketMreq) error {
+	return setsockopt(fd, level, opt, unsafe.Pointer(mreq), unsafe.Sizeof(*mreq))
+}
+
 // SetsockoptSockFprog attaches a classic BPF or an extended BPF program to a
 // socket to filter incoming packets.  See 'man 7 socket' for usage information.
 func SetsockoptSockFprog(fd, level, opt int, fprog *SockFprog) error {