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/socks.h

@@ -43,32 +43,6 @@ int Curl_blockread_all(struct Curl_easy *data,
                        ssize_t buffersize,
                        ssize_t *n);
 
-int Curl_SOCKS_getsock(struct connectdata *conn,
-                       curl_socket_t *sock,
-                       int sockindex);
-/*
- * This function logs in to a SOCKS4(a) proxy and sends the specifics to the
- * final destination server.
- */
-CURLproxycode Curl_SOCKS4(const char *proxy_name,
-                          const char *hostname,
-                          int remote_port,
-                          int sockindex,
-                          struct Curl_easy *data,
-                          bool *done);
-
-/*
- * This function logs in to a SOCKS5 proxy and sends the specifics to the
- * final destination server.
- */
-CURLproxycode Curl_SOCKS5(const char *proxy_name,
-                          const char *proxy_password,
-                          const char *hostname,
-                          int remote_port,
-                          int sockindex,
-                          struct Curl_easy *data,
-                          bool *done);
-
 #if defined(HAVE_GSSAPI) || defined(USE_WINDOWS_SSPI)
 /*
  * This function handles the SOCKS5 GSS-API negotiation and initialization
@@ -77,6 +51,10 @@ CURLcode Curl_SOCKS5_gssapi_negotiate(int sockindex,
                                       struct Curl_easy *data);
 #endif
 
+CURLcode Curl_cfilter_socks_proxy_add(struct Curl_easy *data,
+                                      struct connectdata *conn,
+                                      int sockindex);
+
 #endif /* CURL_DISABLE_PROXY */
 
 #endif  /* HEADER_CURL_SOCKS_H */