New libcurl option to keep sending on error

Add the new option CURLOPT_KEEP_SENDING_ON_ERROR to control whether
sending the request body shall be completed when the server responds
early with an error status code.

This is suitable for manual NTLM authentication.

Reviewed-by: Jay Satiro

Closes https://github.com/curl/curl/pull/904

include/curl/curl.h

@@ -1700,6 +1700,10 @@ typedef enum {
   /* Set TCP Fast Open */
   CINIT(TCP_FASTOPEN, LONG, 244),
 
+  /* Continue to send data if the server responds early with an
+   * HTTP status code >= 300 */
+  CINIT(KEEP_SENDING_ON_ERROR, LONG, 245),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;