[linker] blacklist System.Net.Sockets.MulticastOption fields

 - merge fix for https://bugzilla.xamarin.com/show_bug.cgi?id=21578
   the MulticastOption's 'private IPAddress local' field was
   removed by linker and the app crashed later at runtime.
 - when System.Net.Sockets.MulticastOption object
   is passed to System.Net.Sockets.Socket.SetSocketOption
   and thru it to System.Net.Sockets.Socket.SetSocketOption_internal
   it accesses passed object fields
   (thru few more native calls).

mcs/tools/linker/Descriptors/System.xml

@@ -13,6 +13,7 @@
 		<type fullname="System.Net.SocketAddress" />
 		<type fullname="System.Net.Sockets.LingerOption" />
 		<type fullname="System.Net.Sockets.Socket" />
+		<type fullname="System.Net.Sockets.MulticastOption" preserve="fields" />
 		<type fullname="System.Net.Sockets.Socket/SocketAsyncResult" preserve="fields" />
 		<type fullname="System.Net.Sockets.SocketException" />
 	</assembly>