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/version.c

@@ -522,7 +522,7 @@ curl_version_info_data *curl_version_info(CURLversion stamp)
   Curl_ssl_version(ssl_buffer, sizeof(ssl_buffer));
   version_info.ssl_version = ssl_buffer;
 #ifndef CURL_DISABLE_PROXY
-  if(Curl_ssl->supports & SSLSUPP_HTTPS_PROXY)
+  if(Curl_ssl_supports(NULL, SSLSUPP_HTTPS_PROXY))
     version_info.features |= CURL_VERSION_HTTPS_PROXY;
   else
     version_info.features &= ~CURL_VERSION_HTTPS_PROXY;