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, 64 insertions, 0 deletions

diff --git a/docs/libcurl/opts/CURLOPT_PROXY_CAINFO.3 b/docs/libcurl/opts/CURLOPT_PROXY_CAINFO.3
new file mode 100644
index 000000000..31fc562e8
--- /dev/null
+++ b/docs/libcurl/opts/CURLOPT_PROXY_CAINFO.3
@@ -0,0 +1,64 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  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_CAINFO 3 "16 Nov 2016" "libcurl 7.51.1" "curl_easy_setopt options"
+.SH NAME
+CURLOPT_PROXY_CAINFO \- path to proxy Certificate Authority (CA) bundle
+.SH SYNOPSIS
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PROXY_CAINFO, char *path);
+.SH DESCRIPTION
+TODO: Make this text specific to HTTPS proxy. (Added in 7.XXX)
+Pass a char * to a zero terminated string naming a file holding one or more
+certificates to verify the peer with.
+
+If \fICURLOPT_SSL_VERIFYPEER(3)\fP is zero and you avoid verifying the
+server's certificate, \fICURLOPT_CAINFO(3)\fP need not even indicate an
+accessible file.
+
+This option is by default set to the system path where libcurl's cacert bundle
+is assumed to be stored, as established at build time.
+
+If curl is built against the NSS SSL library, the NSS PEM PKCS#11 module
+(libnsspem.so) needs to be available for this option to work properly.
+
+(iOS and macOS only) If curl is built against Secure Transport, then this
+option is supported for backward compatibility with other SSL engines, but it
+should not be set. If the option is not set, then curl will use the
+certificates in the system and user Keychain to verify the peer, which is the
+preferred method of verifying the peer's certificate chain.
+.SH DEFAULT
+Built-in system specific
+.SH PROTOCOLS
+All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.
+.SH EXAMPLE
+TODO
+.SH AVAILABILITY
+For SSL engines that don't support certificate files the CURLOPT_PROXY_CAINFO option
+is ignored. Refer to https://curl.haxx.se/docs/ssl-compared.html
+.SH RETURN VALUE
+Returns CURLE_OK if the option is supported, CURLE_UNKNOWN_OPTION if not, or
+CURLE_OUT_OF_MEMORY if there was insufficient heap space.
+.SH "SEE ALSO"
+.BR CURLOPT_CAPATH "(3), "
+.BR CURLOPT_SSL_VERIFYPEER "(3), " CURLOPT_SSL_VERIFYHOST "(3), "