Bug 2785: DNS needs to set EDNS options advertising Squid capabilities

Bug 2785: DNS needs to set EDNS options advertising Squid capabilities

... allowing Squid to advertise a larger UDP reply size than 512 bytes.

Internally Squid has a buffer allocated on demand so there is no
practicable limit on individual packets. Network topology and external
software places stricter boundaries on what works and what does not.

Squid does not parse the additional section of replies so for now the
full auto-negotiation EDNS allows is not used. Instead a configuration
option is provided for admin to configure a desirable packet size in
bytes. EDNS defaults to "none" (disabled) until tested in a wider
environment.

Testing so far has brought to light problems with EDNS adverts on A and
IPv4-PTR queries. So support is limited to AAAA and IPv6-PTR queries only.
EDNS compliant resolvers have the option of caching the info between
requests for a short while so this will hopefully leak over to improve
IPv4 responses as well.