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.

include/curl/curl.h

@@ -2592,6 +2592,7 @@ typedef enum {
   CURLVERSION_SECOND,
   CURLVERSION_THIRD,
   CURLVERSION_FOURTH,
+  CURLVERSION_FIFTH,
   CURLVERSION_LAST /* never actually use this */
 } CURLversion;
 
@@ -2600,7 +2601,7 @@ typedef enum {
    meant to be a built-in version number for what kind of struct the caller
    expects. If the struct ever changes, we redefine the NOW to another enum
    from above. */
-#define CURLVERSION_NOW CURLVERSION_FOURTH
+#define CURLVERSION_NOW CURLVERSION_FIFTH
 
 typedef struct {
   CURLversion age;          /* age of the returned struct */
@@ -2628,6 +2629,12 @@ typedef struct {
 
   const char *libssh_version; /* human readable string */
 
+  /* These fields were added in CURLVERSION_FIFTH */
+
+  unsigned int brotli_ver_num; /* Numeric Brotli version
+                                  (MAJOR << 24) | (MINOR << 12) | PATCH */
+  const char *brotli_version; /* human readable string. */
+
 } curl_version_info_data;
 
 #define CURL_VERSION_IPV6         (1<<0)  /* IPv6-enabled */
@@ -2658,6 +2665,7 @@ typedef struct {
                                              for cookie domain verification */
 #define CURL_VERSION_HTTPS_PROXY  (1<<21) /* HTTPS-proxy support built-in */
 #define CURL_VERSION_MULTI_SSL    (1<<22) /* Multiple SSL backends available */
+#define CURL_VERSION_BROTLI       (1<<23) /* Brotli features are present. */
 
  /*
  * NAME curl_version_info()