This uses the brotli external library (https://github.com/google/brotli).
Brotli becomes a feature: additional curl_version_info() bit and
structure fields are provided for it and CURLVERSION_NOW bumped.
Tests 314 and 315 check Brotli content unencoding with correct and
erroneous data.
Some tests are updated to accomodate with the now configuration dependent
parameters of the Accept-Encoding header.
This is implemented as an output streaming stack of unencoders, the last
calling the client write procedure.
New test 230 checks this feature.
Bug: https://github.com/curl/curl/pull/2002
Reported-By: Daniel Bankhead
