http: fix sending of large requests

- refs #11342 where errors with git https interactions
  were observed
- problem was caused by 1st sends of size larger than 64KB
  which resulted in later retries of 64KB only
- limit sending of 1st block to 64KB
- adjust h2/h3 filters to cope with parsing the HTTP/1.1
  formatted request in chunks

- introducing Curl_nwrite() as companion to Curl_write()
  for the many cases where the sockindex is already known

Fixes #11342 (again)
Closes #11803

lib/sendf.h

@@ -51,4 +51,11 @@ CURLcode Curl_write(struct Curl_easy *data,
                     const void *mem, size_t len,
                     ssize_t *written);
 
+/* internal write-function, using sockindex for connection destination */
+CURLcode Curl_nwrite(struct Curl_easy *data,
+                     int sockindex,
+                     const void *buf,
+                     size_t blen,
+                     ssize_t *pnwritten);
+
 #endif /* HEADER_CURL_SENDF_H */