(make_remember_continuation): New function. (Currently not used).
(do_listen): Generalized, so that it can be used by all of
do_simple_listen, do_listen_with_callback and do_listen_with_connection.
(listen_with_callback):  Renamed the improperly named listen_connection.
(listen_with_connection): New command.

* src/io.c (choose_address): Choose one address fron an
addrinfo-list, using a list of preferred address families.
(address_info2sockaddr): Added preference argument. Updated all
calls.

Rev: src/io.c:1.97
Rev: src/io_commands.c:1.30
Rev: src/io_commands.h:1.13