modify reference counting within netmgr

modify reference counting within netmgr

- isc__nmhandle_get() now attaches to the sock in the nmhandle object.
  the caller is responsible for dereferencing the original socket
  pointer when necessary.
- tcpdns listener sockets attach sock->outer to the outer tcp listener
  socket. tcpdns connected sockets attach sock->outerhandle to the handle
  for the tcp connected socket.
- only listener sockets need to be attached/detached directly. connected
  sockets should only be accessed and reference-counted via their
  associated handles.