MINOR: listener: move the DEF_ACCEPT option to the bind_conf

MINOR: listener: move the DEF_ACCEPT option to the bind_conf

This option is set per bind line, and was only set stored when the
address family is AF_INET4 or AF_INET6. That's pointless since it's
used only in tcp_bind_listener() which is only used for such families
as well, so it can now be moved to the bind_conf under the name
BC_O_DEF_ACCEPT.