Add BroadcastIP property to TIdStackLocalAddressIPv4

[rlebeau]

Add BroadcastIP property to TIdStackLocalAddressIPv4. On platforms with getifaddrs() available, simply return the ifaddrs.ifa_broadaddr field if the ifaddrs.ifa_flags field contains the IFF_BROADCAST flag. Otherwise, calculate the broadcast IP manually using the adapter's IP address and subnet mask:

broadcastIP := adapterIP or (not subnetMask);