INCOMPATIBLE CHANGE!

Since it does not seem that anybody has ever even tried to use
the HTTP GET backend (at least not to the point of worrying about
encodings and decoding HTTP entities), and it causes unnecessary
trouble when implementing confget in other languages, kill it.

Relatedly, update the test suite:
- change the JSON schema version to 2.0
- drop the "http_get" enumeration member
- drop the two "setenv" objects, no longer needed
- do not parse the "setenv" specifications or generate code for them