migrate to dhtnet namespace

include/certstore.h

include/connectionmanager.h

include/diffie-hellman.h

include/fileutils.h

include/generic_io.h

include/ice_options.h

include/ice_transport.h

include/ip_utils.h

include/multiplexed_socket.h

include/string_utils.h

include/tls_session.h

include/upnp/mapping.h

include/upnp/upnp_context.h

include/upnp/upnp_control.h

include/upnp/upnp_thread_util.h

src/connectionmanager.cpp

src/fileutils.cpp

src/ice_socket.h

src/ice_transport.cpp

src/ice_transport_factory.h