Move listener socket handling to libcomm-listener.la

Move listener socket handling to libcomm-listener.la

* Renamed AcceptFD to ListenStateData
* listener sockets 'owned' by the code which opened them
  - FtpStateData owns the FTP data sockets
  - config HTTP settings own the http_port/https_port listeners.
* Removed fdc_table - replaced with Comm::CurrentListenerSockets.
  - reasonable memory savings (default 32-bit build only 8 KB, large production 64-bit build save 768 KB)
  - initialized in one simple fast step.
  - changes only made by ListenStateData constructor/destructor
* HotConf ready listener sockets (depends only on further config changes to happen)