split AddressableBSDSocketProtocol of BSDSocketProtocol

added listen to ConnectedCommunicationPolicy
added primitive doxygen documentation
cleaned up naming of some files
fixed INetProtocol.cc (missing const modifiers)
removed SocketProtcolHelper in favor of directly inheriting SocketProtocol virtually
moved implementation of TCPv4SocketProtocol virtual methods into the base classes
added rudimentary testing of the TCPv4SocketHandle protocol interface
finalized PacketSocketHandle