UDP connection handler for DNS/TCP fallback

[alalamav]

• Implements a UDP connection handler that responds to DNS queries with a truncated bit in order for the query to be retried over TCP.
  • This is useful for servers that do not support UDP, and/or networks that block UDP traffic to non-standard ports.
  • While TCP support is advisory in DNS resolvers, we have found that major resolvers support it in practice. Additionally, DNS clients in all major operating systems support the retry behavior when receiving a truncated response.
  • See RFC 1123, section 6.1.3.2 for more details.
• Exposes a flag -dnsFallback to enable this behavior by overriding the specified UDP proxy handler.
• Builds the module when "dnsfallback" is present in $BUILD_TAGS.
• Sets a default value for $BUILD_TGAS, so it can be customized from the command line.

[eycorsican]

Thx.