proxy: Support HTTPS proxy and SOCKS+HTTP(s)

HTTPS proxies:

An HTTPS proxy receives all transactions over an SSL/TLS connection. Once a
secure connection with the proxy is established, the user agent uses the proxy
as usual, including sending CONNECT requests to instruct the proxy to establish
a [usually secure] TCP tunnel with an origin server. HTTPS proxies protect
nearly all aspects of user-proxy communications as opposed to HTTP proxies that
receive all requests (including CONNECT requests) in vulnerable clear text.

With HTTPS proxies, it is possible to have two concurrent _nested_ SSL/TLS
sessions: the "outer" one between the user agent and the proxy and the "inner"
one between the user agent and the origin server (through the proxy). This
change adds supports for such nested sessions as well.

The secure connection with the proxy requires its own set of the usual
SSL/TLS-related options (their descriptions need polishing):

  --proxy-cacert FILE        CA certificate to verify peer against
  --proxy-capath DIR         CA directory to verify peer against
  --proxy-cert CERT[:PASSWD] Client certificate file and password
  --proxy-cert-type TYPE     Certificate file type (DER/PEM/ENG)
  --proxy-ciphers LIST       SSL ciphers to use
  --proxy-crlfile FILE       Get a CRL list in PEM format from the given file
  --proxy-insecure           Allow connections to SSL sites without certs
  --proxy-key KEY            Private key file name
  --proxy-key-type TYPE      Private key file type (DER/PEM/ENG)
  --proxy-pass PASS          Pass phrase for the private key
  --proxy-ssl-allow-beast    Allow security flaw to improve interop
  --proxy-sslv2              Use SSLv2
  --proxy-sslv3              Use SSLv3
  --proxy-tlsv1              Use TLSv1
  --proxy-tlsuser USER       TLS username
  --proxy-tlspassword STRING TLS password
  --proxy-tlsauthtype STRING TLS authentication type (default SRP)

All --proxy-foo options are independent from their --foo counterparts, except
--proxy-crlfile defaults to --crlfile and --proxy-capath defaults to --capath.

Curl now also supports %{proxy_ssl_verify_result} --write-out variable,
similar to the existing %{ssl_verify_result} variable.

SOCKS proxy + HTTP/HTTPS proxy combination:

If both --socks* and --proxy options are given, Curl first connects to the
SOCKS proxy and then connects (through SOCKS) to the HTTP or HTTPS proxy.

lib/http_proxy.c

@@ -37,19 +37,57 @@
 #include "connect.h"
 #include "curl_printf.h"
 #include "curlx.h"
+#include "vtls/vtls.h"
 
 #include "curl_memory.h"
 /* The last #include file should be: */
 #include "memdebug.h"
 
-CURLcode Curl_proxy_connect(struct connectdata *conn)
+/*
+ * Perform SSL initialization for HTTPS proxy.  Sets
+ * proxy_ssl_connected connection bit when complete.  Can be
+ * called multiple times.
+ */
+static CURLcode https_proxy_connect(struct connectdata *conn, int sockindex)
 {
+#ifdef USE_SSL
+  CURLcode result = CURLE_OK;
+  DEBUGASSERT(conn->http_proxy.proxytype == CURLPROXY_HTTPS);
+  if(!conn->bits.proxy_ssl_connected[sockindex]) {
+    /* perform SSL initialization for this socket */
+    result =
+      Curl_ssl_connect_nonblocking(conn, sockindex,
+                                   &conn->bits.proxy_ssl_connected[sockindex]);
+    if(result)
+      conn->bits.close = TRUE; /* a failed connection is marked for closure to
+                                  prevent (bad) re-use or similar */
+  }
+  return result;
+#else
+  return CURLE_NOT_BUILT_IN;
+#endif
+}
+
+CURLcode Curl_proxy_connect(struct connectdata *conn, int sockindex)
+{
+  if(conn->http_proxy.proxytype == CURLPROXY_HTTPS) {
+    const CURLcode result = https_proxy_connect(conn, sockindex);
+    if(result)
+      return result;
+    if(!conn->bits.proxy_ssl_connected[sockindex])
+      return result; /* wait for HTTPS proxy SSL initialization to complete */
+  }
+
   if(conn->bits.tunnel_proxy && conn->bits.httpproxy) {
 #ifndef CURL_DISABLE_PROXY
     /* for [protocol] tunneled through HTTP proxy */
     struct HTTP http_proxy;
     void *prot_save;
     CURLcode result;
+    const char * const host = sockindex == SECONDARYSOCKET ?
+      conn->secondaryhostname : conn->host.name;
+    const int port = sockindex == SECONDARYSOCKET ? conn->secondary_port :
+      conn->remote_port;
 
     /* BLOCKING */
     /* We want "seamless" operations through HTTP proxy tunnel */
@@ -67,8 +105,7 @@ CURLcode Curl_proxy_connect(struct connectdata *conn)
     memset(&http_proxy, 0, sizeof(http_proxy));
     conn->data->req.protop = &http_proxy;
     connkeep(conn, "HTTP proxy CONNECT");
-    result = Curl_proxyCONNECT(conn, FIRSTSOCKET,
-                               conn->host.name, conn->remote_port, FALSE);
+    result = Curl_proxyCONNECT(conn, sockindex, host, port, FALSE);
     conn->data->req.protop = prot_save;
     if(CURLE_OK != result)
       return result;
@@ -152,7 +189,7 @@ CURLcode Curl_proxyCONNECT(struct connectdata *conn,
         char *host=(char *)"";
         const char *proxyconn="";
         const char *useragent="";
-        const char *http = (conn->proxytype == CURLPROXY_HTTP_1_0) ?
+        const char *http = (conn->http_proxy.proxytype == CURLPROXY_HTTP_1_0) ?
           "1.0" : "1.1";
         char *hostheader= /* host:port with IPv6 support */
           aprintf("%s%s%s:%hu", conn->bits.ipv6_ip?"[":"",