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.

A secure connection with a proxy requires its own set of the usual SSL
options (their actual descriptions differ and need polishing, see TODO):

  --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 file
  --proxy-insecure           Allow connections to proxies with bad 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 which defaults to --crlfile and --proxy-capath
which defaults to --capath.

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

Supported backends: OpenSSL, GnuTLS, and NSS.

* A 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.

TODO: Update documentation for the new APIs and --proxy-* options.
Look for "Added in 7.XXX" marks.

1 files changed, 82 insertions, 0 deletions

diff --git a/docs/libcurl/opts/CURLOPT_PROXY_SSL_VERIFYPEER.3 b/docs/libcurl/opts/CURLOPT_PROXY_SSL_VERIFYPEER.3
new file mode 100644
index 000000000..294c77b37
--- /dev/null
+++ b/docs/libcurl/opts/CURLOPT_PROXY_SSL_VERIFYPEER.3
@@ -0,0 +1,82 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 1998 - 2016, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" *
+.\" * This software is licensed as described in the file COPYING, which
+.\" * you should have received as part of this distribution. The terms
+.\" * are also available at https://curl.haxx.se/docs/copyright.html.
+.\" *
+.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+.\" * copies of the Software, and permit persons to whom the Software is
+.\" * furnished to do so, under the terms of the COPYING file.
+.\" *
+.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+.\" * KIND, either express or implied.
+.\" *
+.\" **************************************************************************
+.\"
+.TH CURLOPT_PROXY_SSL_VERIFYPEER 3 "16 Nov 2016" "libcurl 7.51.1" "curl_easy_setopt options"
+.SH NAME
+CURLOPT_PROXY_SSL_VERIFYPEER \- verify the proxy peer's SSL certificate
+.SH SYNOPSIS
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PROXY_SSL_VERIFYPEER, long verify);
+.SH DESCRIPTION
+TODO: Make this text specific to HTTPS proxy. (Added in 7.XXX)
+Pass a long as parameter to enable or disable.
+
+This option determines whether curl verifies the authenticity of the peer's
+certificate. A value of 1 means curl verifies; 0 (zero) means it doesn't.
+
+When negotiating a TLS or SSL connection, the server sends a certificate
+indicating its identity.  Curl verifies whether the certificate is authentic,
+i.e. that you can trust that the server is who the certificate says it is.
+This trust is based on a chain of digital signatures, rooted in certification
+authority (CA) certificates you supply.  curl uses a default bundle of CA
+certificates (the path for that is determined at build time) and you can
+specify alternate certificates with the \fICURLOPT_CAINFO(3)\fP option or the
+\fICURLOPT_CAPATH(3)\fP option.
+
+When \fICURLOPT_SSL_VERIFYPEER(3)\fP is enabled, and the verification fails to
+prove that the certificate is authentic, the connection fails.  When the
+option is zero, the peer certificate verification succeeds regardless.
+
+Authenticating the certificate is not enough to be sure about the server. You
+typically also want to ensure that the server is the server you mean to be
+talking to.  Use \fICURLOPT_SSL_VERIFYHOST(3)\fP for that. The check that the
+host name in the certificate is valid for the host name you're connecting to
+is done independently of the \fICURLOPT_SSL_VERIFYPEER(3)\fP option.
+
+WARNING: disabling verification of the certificate allows bad guys to
+man-in-the-middle the communication without you knowing it. Disabling
+verification makes the communication insecure. Just having encryption on a
+transfer is not enough as you cannot be sure that you are communicating with
+the correct end-point.
+.SH DEFAULT
+By default, curl assumes a value of 1.
+.SH PROTOCOLS
+All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.
+.SH EXAMPLE
+.nf
+CURL *curl = curl_easy_init();
+if(curl) {
+  curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
+
+  /* Set the default value: strict certificate check please */
+  curl_easy_setopt(curl, CURLOPT_PROXY_SSL_VERIFYPEER, 1L);
+
+  curl_easy_perform(curl);
+}
+.fi
+.SH AVAILABILITY
+If built TLS enabled.
+.SH RETURN VALUE
+Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+.SH "SEE ALSO"
+.BR CURLOPT_SSL_VERIFYHOST "(3), "