Merge of r4993, r4994, r4997, r5000: geo ipv6 support.

Merge of r4993, r4994, r4997, r5000: geo ipv6 support.

*) Geo: IPv6 support.  The "ranges" mode is still limited to IPv4 only.

*) Geo: properly initialize ngx_cidr_t when dealing with "default".

*) Geo: made "default" affect both IPv4 and IPv6 when using prefixes.
   Previously, "default" was equivalent to specifying 0.0.0.0/0, now
   it's equivalent to specifying both 0.0.0.0/0 and ::/0 (if support
   for IPv6 is enabled) with the same value.

*) Geo: improved code readability.