VIP33: Socket.* CLI commands

First revision

socket.open
  Attempts to open all configured incoming sockets (= -a arguments)
  Success if they all open
  Failure if any fail to open, all sockets left closed.

socket.close
  Closes all configured incoming sockets
  Always success.

socket.list
  Output:
     name state fd bound_address a_arg_possibly_filtered_by_transport

  for instance when open:
     a0 open 8 0.0.0.0:3939 :0,PROXY
     a0 open 9 [::]:4029 :0,PROXY

  example when closed:
     a0 closed - - :0,PROXY

This will be the initial behavior without arguments, and commands will be refined when optional arguments are defined.

Second revision

TBD

See https://github.com/varnishcache/varnish-cache/pull/3959 for discussions.