udp6: Avoid special handling of unknown local addresses

[chrysn]

With the currently proposed resolution of #319, looks like we're running a UDP socket connect syscall per peer to which we send something for the first time.

As that connected socket will also have a local address associated with it, we may manage to simplify some things where previously we've lugged around a None. (But maybe that's not a good idea because it's cross-layer ... not sure off my head).