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/http.h

@@ -94,10 +94,6 @@ CURLcode Curl_dynhds_add_custom(struct Curl_easy *data,
                                 bool is_connect,
                                 struct dynhds *hds);
 
-CURLcode Curl_http_compile_trailers(struct curl_slist *trailers,
-                                    struct dynbuf *buf,
-                                    struct Curl_easy *handle);
-
 void Curl_http_method(struct Curl_easy *data, struct connectdata *conn,
                       const char **method, Curl_HttpReq *);
 CURLcode Curl_http_useragent(struct Curl_easy *data);
@@ -112,8 +108,8 @@ CURLcode Curl_transferencode(struct Curl_easy *data);
 CURLcode Curl_http_body(struct Curl_easy *data, struct connectdata *conn,
                         Curl_HttpReq httpreq,
                         const char **teep);
-CURLcode Curl_http_req_send(struct Curl_easy *data,
-                            struct dynbuf *r, Curl_HttpReq httpreq);
+CURLcode Curl_http_req_complete(struct Curl_easy *data,
+                                struct dynbuf *r, Curl_HttpReq httpreq);
 bool Curl_use_http_1_1plus(const struct Curl_easy *data,
                            const struct connectdata *conn);
 #ifndef CURL_DISABLE_COOKIES