reserve half of client lists for ipv4 (take #2)

also rename client_in_list() to client_or_ip_port_in_list() - it does check for an identical ip/port and replaces the client_id, recycling the entry

DHT.c:
- addto_lists(): convert ipv4-in-ipv6 mapped to ipv4 (and reformat argument list layout to the usual)
- replace_bad(), replace_good(): if IPv6, only insert into the upper half of the given list
- rename client_in_list() to client_or_ip_port_in_list()