Allow enabling/disabling IPv4 / 6

[willstott101]

per-Responder or per-Service ?

Inspired by #3

[willstott101]

@ryanpresciense Have you got a real use-case for this?

[ashthespy]

One usecase for this might be - librespot-org/librespot#67

[willstott101]

Interesting... perhaps making it possible to allow failure to bind to one of the two interfaces, would be enough.

Perhaps:

• let responder = Responder::new()
• responder.register(...) Can be before or after ipv4/6 binding
• handle.spawn(responder.ipv4_task()?)
• handle.spawn(responder.ipv6_task()?)

User-code can then decide to let things slide or not.