lib: connection filters (cfilter) addition to curl:

- general construct/destroy in connectdata
 - default implementations of callback functions
 - connect: cfilters for connect and accept
 - socks: cfilter for socks proxying
 - http_proxy: cfilter for http proxy tunneling
 - vtls: cfilters for primary and proxy ssl
 - change in general handling of data/conn
 - Curl_cfilter_setup() sets up filter chain based on data settings,
   if none are installed by the protocol handler setup
 - Curl_cfilter_connect() boot straps filters into `connected` status,
   used by handlers and multi to reach further stages
 - Curl_cfilter_is_connected() to check if a conn is connected,
   e.g. all filters have done their work
 - Curl_cfilter_get_select_socks() gets the sockets and READ/WRITE
   indicators for multi select to work
 - Curl_cfilter_data_pending() asks filters if the have incoming
   data pending for recv
 - Curl_cfilter_recv()/Curl_cfilter_send are the general callbacks
   installed in conn->recv/conn->send for io handling
 - Curl_cfilter_attach_data()/Curl_cfilter_detach_data() inform filters
   and addition/removal of a `data` from their connection
 - adding vtl functions to prevent use of Curl_ssl globals directly
   in other parts of the code.

Reviewed-by: Daniel Stenberg
Closes #9855

lib/sendf.c

@@ -38,6 +38,7 @@
 
 #include "urldata.h"
 #include "sendf.h"
+#include "cfilters.h"
 #include "connect.h"
 #include "vtls/vtls.h"
 #include "vssh/ssh.h"
@@ -159,7 +160,7 @@ static CURLcode pre_receive_plain(struct Curl_easy *data,
      performed before every send() if any incoming data is
      available. However, skip this, if buffer is already full. */
   if((conn->handler->protocol&PROTO_FAMILY_HTTP) != 0 &&
-     conn->recv[num] == Curl_recv_plain &&
+     conn->recv[num] == Curl_cfilter_recv &&
      (!psnd->buffer || bytestorecv)) {
     const int readymask = Curl_socket_check(sockfd, CURL_SOCKET_BAD,
                                             CURL_SOCKET_BAD, 0);
@@ -718,11 +719,14 @@ CURLcode Curl_read(struct Curl_easy *data,   /* transfer */
 
   nread = conn->recv[num](data, num, buffertofill, bytesfromsocket, &result);
   if(nread < 0)
-    return result;
+    goto out;
 
   *n += nread;
-
-  return CURLE_OK;
+  result = CURLE_OK;
+out:
+  /* DEBUGF(infof(data, "Curl_read(handle=%p) -> %d, nread=%ld",
+        data, result, nread)); */
+  return result;
 }
 
 /* return 0 on success */