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/Makefile.inc

@@ -74,6 +74,7 @@ LIB_VTLS_HFILES =           \
   vtls/schannel.h           \
   vtls/sectransp.h          \
   vtls/vtls.h               \
+  vtls/vtls_int.h           \
   vtls/wolfssl.h            \
   vtls/x509asn1.h
 
@@ -105,6 +106,7 @@ LIB_CFILES =         \
   base64.c           \
   bufref.c           \
   c-hyper.c          \
+  cfilters.c         \
   conncache.c        \
   connect.c          \
   content_encoding.c \
@@ -227,6 +229,7 @@ LIB_HFILES =         \
   asyn.h             \
   bufref.h           \
   c-hyper.h          \
+  cfilters.h         \
   conncache.h        \
   connect.h          \
   content_encoding.h \