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

tests/data/test513

@@ -34,14 +34,6 @@ http://%HOSTIP:%HTTPPORT/%TESTNUMBER
 # Verify data after the test has been "shot"
 <verify>
 <protocol>
-%if !hyper
-POST /%TESTNUMBER HTTP/1.1
-Host: %HOSTIP:%HTTPPORT
-Accept: */*
-Content-Length: 1
-Content-Type: application/x-www-form-urlencoded
-
-%endif
 </protocol>
 # 42 - aborted by callback
 <errorcode>

tests/data/test579

@@ -77,6 +77,8 @@ http://%HOSTIP:%HTTPPORT/%TESTNUMBER %LOGDIR/ip%TESTNUMBER
 <verify>
 <file name="%LOGDIR/ip%TESTNUMBER">
 Progress callback called with UL 0 out of 0
+Progress callback called with UL 5 out of 0
+Progress callback called with UL 0 out of 0
 Progress callback called with UL 8 out of 0
 Progress callback called with UL 16 out of 0
 Progress callback called with UL 26 out of 0