Send discovery requests over link-local addresses in case multicast is unavailable

[seamlik]

This usually happens when 2 devices are connected directly without a router. For example:

• 2 Android devices connected via the hotspot provided by one of them
• 2 laptops connected via an ethernet cable

In these scenarios, the user might try to transfer large files between devices, and there will be only 2 devices in the LAN, 1 being the gateway. Since multicast is likely not implemented or disabled, we can use the gateway IP as if it is the multicast discovery IP.