Streamlined listener/select/accept handling:

  For ITS#2482:
  added sl_is_mute flag to Listener - when set, the listener is ignored
  if accept() on a listener fails with EMFILE, set the sl_is_mute flag
  when removing a connection, if emfile != 0, look for a listener to unmute

  For ITS#2374:
  if global_idletimeout is set and there are active sessions, use a timeout
  on the select() to actively detect idle sessions.