options: added basic SASL XOAUTH2 support

Added the ability to specify an XOAUTH2 bearer token [RFC6750] via the
option CURLOPT_XOAUTH2_BEARER for authentication using RFC6749 "OAuth
2.0 Authorization Framework".

include/curl/curl.h

@@ -1553,6 +1553,9 @@ typedef enum {
    * prototype defines. (Deprecates CURLOPT_PROGRESSFUNCTION) */
   CINIT(XFERINFOFUNCTION, FUNCTIONPOINT, 219),
 
+  /* The XOAUTH2 bearer token */
+  CINIT(XOAUTH2_BEARER, OBJECTPOINT, 220),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;
 

include/curl/typecheck-gcc.h

@@ -264,6 +264,7 @@ _CURL_WARNING(_curl_easy_getinfo_err_curl_slist,
    (option) == CURLOPT_RTSP_SESSION_ID ||                                     \
    (option) == CURLOPT_RTSP_STREAM_URI ||                                     \
    (option) == CURLOPT_RTSP_TRANSPORT ||                                      \
+   (option) == CURLOPT_XOAUTH2_BEARER ||                                      \
    0)
 
 /* evaluates to true if option takes a curl_write_callback argument */