Add specific support for broadcast

[tjguk]

At present we use a broadcast socket as part of the beacon mechanism but it might be useful to support it specifically within networkzero, eg wait_for_broadcast / send_broadcast.

The upsides:

• It's a standard mechanism
• It can be useful especially for dynamic discovery

The downsides:

• It's likely to interfere with the beacon (at least: we'd have to work quite hard to make sure it didn't)
• It adds a little clutter to the API and it's just about possible to use the discovery mechanism itself for some of the same purposes