Enable EDNS for DNS A queries and reverse IPv4 lookups (#1864)

Enable EDNS for DNS A queries and reverse IPv4 lookups (#1864)

This change brings Squid code in sync with existing dns_packet_max
directive documentation and allows admins to enable a useful performance
optimization for still very popular IPv4-related DNS queries.

An enabled dns_packet_max feature may now break Squid compatibility with
more buggy nameservers (that appeared to "work" before), but we should
not penalize many Squid deployments (or complicate configuration and
spend a lot of development time) to accommodate a few exceptional ones,
at least until such heroic efforts are proven to be necessary.