lib: Curl_read/Curl_write clarifications

- replace `Curl_read()`, `Curl_write()` and `Curl_nwrite()` to
  clarify when and at what level they operate
- send/recv of transfer related data is now done via
  `Curl_xfer_send()/Curl_xfer_recv()` which no longer has
  socket/socketindex as parameter. It decides on the transfer
  setup of `conn->sockfd` and `conn->writesockfd` on which
  connection filter chain to operate.
- send/recv on a specific connection filter chain is done via
  `Curl_conn_send()/Curl_conn_recv()` which get the socket index
  as parameter.
- rename `Curl_setup_transfer()` to `Curl_xfer_setup()` for
  naming consistency
- clarify that the special CURLE_AGAIN hangling to return
  `CURLE_OK` with length 0 only applies to `Curl_xfer_send()`
  and CURLE_AGAIN is returned by all other send() variants.
- fix a bug in websocket `curl_ws_recv()` that mixed up data
  when it arrived in more than a single chunk (to be made
  into a sperate PR, also)

Added as documented [in
CLIENT-READER.md](5b1f31dfba/docs/CLIENT-READERS.md).

- old `Curl_buffer_send()` completely replaced by new `Curl_req_send()`
- old `Curl_fillreadbuffer()` replaced with `Curl_client_read()`
- HTTP chunked uploads are now formatted in a client reader added when
  needed.
- FTP line-end conversions are done in a client reader added when
  needed.
- when sending requests headers, remaining buffer space is filled with
  body data for sending in "one go". This is independent of the request
  body size. Resolves #12938 as now small and large requests have the
  same code path.

Changes done to test cases:

- test513: now fails before sending request headers as this initial
  "client read" triggers the setup fault. Behaves now the same as in
  hyper build
- test547, test555, test1620: fix the length check in the lib code to
  only fail for reads *smaller* than expected. This was a bug in the
  test code that never triggered in the old implementation.

Closes #12969

lib/file.c

@@ -293,10 +293,10 @@ static CURLcode file_upload(struct Curl_easy *data)
   CURLcode result = CURLE_OK;
   char *xfer_buf;
   size_t xfer_blen;
-  char *uphere_save;
   curl_off_t bytecount = 0;
   struct_stat file_stat;
   const char *sendbuf;
+  bool eos = FALSE;
 
   /*
    * Since FILE: doesn't do the full init, we need to provide some extra
@@ -340,21 +340,16 @@ static CURLcode file_upload(struct Curl_easy *data)
     data->state.resume_from = (curl_off_t)file_stat.st_size;
   }
 
-  /* Yikes! Curl_fillreadbuffer uses data->req.upload_fromhere to READ
-   * client data to! Please, someone fix... */
-  uphere_save = data->req.upload_fromhere;
-
   result = Curl_multi_xfer_buf_borrow(data, &xfer_buf, &xfer_blen);
   if(result)
     goto out;
 
-  while(!result) {
+  while(!result && !eos) {
     size_t nread;
     ssize_t nwrite;
     size_t readcount;
 
-    data->req.upload_fromhere = xfer_buf;
-    result = Curl_fillreadbuffer(data, xfer_blen, &readcount);
+    result = Curl_client_read(data, xfer_buf, xfer_blen, &readcount, &eos);
     if(result)
       break;
 
@@ -401,7 +396,6 @@ static CURLcode file_upload(struct Curl_easy *data)
 out:
   close(fd);
   Curl_multi_xfer_buf_release(data, xfer_buf);
-  data->req.upload_fromhere = uphere_save;
 
   return result;
 }