Similarly to what we do in package xio (see 7ad867a3 "xio: Add
Reader/Writer/... interfaces - io analogs that add support for
contexts"), add utilities to bind xnet.Listener to context (giving
net.Listener), and, on the other hand, adapt uncontextified net.Listener
to xnet.Listener that handles cancellation.

This utilities are needed because a lot of third-party code wants to
work with net.Listener interface only.

Contrary to xio, we can make it working reliably in both ways (because
Accept already buffers accepting connections starting at OS level).