Support builds on hosts which don't have mono installed
Allow udp sockets to set the IP_MULTICAST_IF socket option.  This allows tests to run on loopback on Mac.
Fix incorrect merge
Disable OsNetworkSocketSetMulticastIf() for Linux.  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=607979 suggests the implementation won't work against older kernels.  (We could alter the function signature amd pass an im_reqn to make it work on all linux versions.  It doesn't seem worth the extra disruption currently when only Mac benefits from this sockopt.)