ShortcutMac doesn't allow enabling multicast support in the interfaceEntry using a parameter

[rhornig]

ShortcutMac should support also multicast (as it is supposed to be a drop-in replacement for other types of interfaces). Currently it is harcoded to be a broadcast interface, which should be also configurable.

void ShortcutMac::configureNetworkInterface()
{
    MacAddress address = parseMacAddressParameter(par("address"));
    shortcutMacs[address] = this;
    networkInterface->setDatarate(bitrate);
    networkInterface->setMacAddress(address);
    networkInterface->setInterfaceToken(address.formInterfaceIdentifier());
    networkInterface->setMtu(par("mtu"));
    networkInterface->setMulticast(false);
    networkInterface->setBroadcast(true);
}