HTTP: implement Brotli content encoding

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.

src/tool_help.c

@@ -499,6 +499,7 @@ static const struct feat feats[] = {
   {"NTLM_WB",        CURL_VERSION_NTLM_WB},
   {"SSL",            CURL_VERSION_SSL},
   {"libz",           CURL_VERSION_LIBZ},
+  {"brotli",         CURL_VERSION_BROTLI},
   {"CharConv",       CURL_VERSION_CONV},
   {"TLS-SRP",        CURL_VERSION_TLSAUTH_SRP},
   {"HTTP2",          CURL_VERSION_HTTP2},