Draft: PNHellos need to be sent on all interfaces

Currently PNHello's are sent out to just one interface: The kernel chooses one interface when sending to ff02::1 without specifying a particular interface.

This results in some links never being discovered as no PNHello's are sent over them.

This MR currently includes a massive hack to fix this:

• Before sending a PNHello the ip command is called to get all available interfaces.
• Then the packet is sent to all of them in a single loop

In the future the implementation should keep track of all available interfaces (most likely via a netlink socket).