internet: sgi::hash_map is non-standard and should be substituted with std::unordered_map

Thanks to @Gabrielcarvfer for the heads-up in #372 (closed)

sgi::hash_map is really old, and its support is "legacy" at best. There's no real reason to keep it, as std::unordered_map is standard, and it does the very same.

Fortunately, sgi::hash_map is only used in NdiscCache and ArpCache.