The ip_anon module masks the host part of the client address.
$remote_addr_anon and $binary_remote_addr_anon carry the result, and
ip_anon_v4_prefix and ip_anon_v6_prefix say how much of the address is
kept, for a log or a limit key that is not meant to name anybody.  An
IPv4-mapped address is masked with the IPv4 prefix, so a dual stack
listener does not log the client's four bytes.

This port only installs the module sources.  The module itself is built by
the nginx ports when their IP_ANON option is enabled.
