We already handle cancellation in Dial, but Accepting was out of luck
until now. This makes it more difficult for clients to implement and
wrap acceptors where they need to handle cancellations. This also makes
it possible for a test or program to get stuck in Accept loop if it is
not careful enough to manually handle ctx cancel around Accept calls.

-> Fix it in one place - here, in xnet - so that users are offloaded
from all this and can just call Accept(ctx) and rely on underlying
implementation to handle ctx cancel.

This patch:

- introduces xnet.Listener interface, which is like net.Listener, but
  Accept goes with ctx argument.
- changes Networker.Listen signature to return xnet.Listener instead of
  net.Listener. While we are here - changing it - also add ctx argument
  to Listen call itself.
- Adds listenerCtx - which, given net.Listener, provides xnet.Listener
  by wrapping some logic around original.
- Adapts NetPlain, NetTLS and NetTrace to provide updated interface.

We'll fix up everything in other packages to match/use updated interface
in the next patch.