sasl: Implement SASL authorisation identity via CURLOPT_SASL_AUTHZID

Added the ability for the calling program to specify the authorisation
identity (authzid), the identity to act as, in addition to the
authentication identity (authcid) and password when using SASL PLAIN
authentication.

Fixed #3653
Closes #3790

include/curl/curl.h

@@ -1921,6 +1921,9 @@ typedef enum {
   /* maximum age of a connection to consider it for reuse (in seconds) */
   CINIT(MAXAGE_CONN, LONG, 288),
 
+  /* SASL authorisation identity */
+  CINIT(SASL_AUTHZID, STRINGPOINT, 289),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;
 

include/curl/typecheck-gcc.h

@@ -309,6 +309,7 @@ _CURL_WARNING(_curl_easy_getinfo_err_curl_off_t,
    (option) == CURLOPT_RTSP_SESSION_ID ||                                     \
    (option) == CURLOPT_RTSP_STREAM_URI ||                                     \
    (option) == CURLOPT_RTSP_TRANSPORT ||                                      \
+   (option) == CURLOPT_SASL_AUTHZID ||                                        \
    (option) == CURLOPT_SERVICE_NAME ||                                        \
    (option) == CURLOPT_SOCKS5_GSSAPI_SERVICE ||                               \
    (option) == CURLOPT_SSH_HOST_PUBLIC_KEY_MD5 ||                             \