Formalize toggle override for non-Tor applications that follow RFC 7686

Suggestion to formalize environment variable name ALLOW_DOT_ONION (or other) to be set to toggle and enable DNS lookup of the .onion TLD where an application otherwise would block such DNS lookup.

This would be useful in transparent proxy setups where you don't want to or cannot do SOCKS proxying.

RFC 7686 states that:

Applications that do not implement the Tor protocol SHOULD generate an error upon the use of .onion and SHOULD NOT perform a DNS lookup.

An example of a common application that refuse to resolve the .onion TLD is curl, since 0ae0abb and even earlier, if built with --enable-ares, since c-ares 955df98. Currently no override is available for curl or c-ares.

An example of an application that refuse to resolve the .onion TLD by default is Firefox, where it is available as a toggle in about:config by setting the network.dns.blockDotOnion boolean to false.