Support transport over IPv6.

We need multiple UDP sockets, for each address family required, with the
knock-on complexity to the sending and receiving code.

[[REBASE NOTE: this commit commutes syntatically with the adns_r_addr
enhancement patch, but won't actually work.  Before the series is
submitted for review, it wants to be hoisted /after/ that patch.  This
note can then be removed.]]

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>