makeIPPorts returns a slice of valid, non-zero at most cap AddrPorts. The first element may be origipp AddrPort, if realips is empty or contains only unspecified IPs. or maybeIncludeOrig is true and origipp's IP family is included in dialer's current config.
(ips []netip.Addr, origipp netip.AddrPort, maybeIncludeOrig bool, cap int)