cf-socket: improvements in socket I/O handling

- Curl_write_plain/Curl_read_plain have been eliminated. Last code use
  now uses Curl_conn_send/recv so that requests use conn->send/revc
  callbacks which defaults to cfilters use.
- Curl_recv_plain/Curl_send_plain have been internalized in cf-socket.c.
- USE_RECV_BEFORE_SEND_WORKAROUND (active on Windows) has been moved
  into cf-socket.c. The pre_recv buffer is held at the socket filter
  context.  `postponed_data` structures have been removed from
  `connectdata`.
- the hanger in HTTP/2 request handling was a result of read buffering
  on all sends and the multi handling is not prepared for this. The
  following happens:

   - multi preforms on a HTTP/2 easy handle
   - h2 reads and processes data
   - this leads to a send of h2 data
   - which receives and buffers before the send
   - h2 returns
   - multi selects on the socket, but no data arrives (its in the buffer already)
   the workaround now receives data in a loop as long as there is something in
   the buffer. The real fix would be for multi to change, so that `data_pending`
   is evaluated before deciding to wait on the socket.

io_buffer, optional, in cf-socket.c, http/2 sets state.drain if lower
filter have pending data.

This io_buffer is only available/used when the
-DUSE_RECV_BEFORE_SEND_WORKAROUND is active, e.g. on Windows
configurations. It also maintains the original checks on protocol
handler being HTTP and conn->send/recv not being replaced.

The HTTP/2 (nghttp2) cfilter now sets data->state.drain when it finds
out that the "lower" filter chain has still pending data at the end of
its IO operation. This prevents the processing from becoming stalled.

Closes #10280

lib/sendf.h

@@ -40,20 +40,6 @@
 CURLcode Curl_client_write(struct Curl_easy *data, int type, char *ptr,
                            size_t len) WARN_UNUSED_RESULT;
 
-bool Curl_recv_has_postponed_data(struct connectdata *conn, int sockindex);
-
-/* internal read-function, does plain socket only */
-CURLcode Curl_read_plain(struct Curl_easy *data,
-                         curl_socket_t sockfd,
-                         char *buf,
-                         size_t sizerequested,
-                         ssize_t *n);
-
-ssize_t Curl_recv_plain(struct Curl_easy *data, int num, char *buf,
-                        size_t len, CURLcode *code);
-ssize_t Curl_send_plain(struct Curl_easy *data, int num,
-                        const void *mem, size_t len, CURLcode *code);
-
 /* internal read-function, does plain socket, SSL and krb4 */
 CURLcode Curl_read(struct Curl_easy *data, curl_socket_t sockfd,
                    char *buf, size_t buffersize,
@@ -65,10 +51,4 @@ CURLcode Curl_write(struct Curl_easy *data,
                     const void *mem, size_t len,
                     ssize_t *written);
 
-/* internal write-function, does plain sockets ONLY */
-CURLcode Curl_write_plain(struct Curl_easy *data,
-                          curl_socket_t sockfd,
-                          const void *mem, size_t len,
-                          ssize_t *written);
-
 #endif /* HEADER_CURL_SENDF_H */