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.

docs/libcurl/curl_easy_setopt.3

@@ -159,10 +159,14 @@ Protocols to allow redirects to. See \fICURLOPT_REDIR_PROTOCOLS(3)\fP
 Default protocol. See \fICURLOPT_DEFAULT_PROTOCOL(3)\fP
 .IP CURLOPT_PROXY
 Proxy to use. See \fICURLOPT_PROXY(3)\fP
+.IP CURLOPT_SOCKS_PROXY
+Socks proxy to use. See \fICURLOPT_SOCKS_PROXY(3)\fP
 .IP CURLOPT_PROXYPORT
 Proxy port to use. See \fICURLOPT_PROXYPORT(3)\fP
 .IP CURLOPT_PROXYTYPE
 Proxy type. See \fICURLOPT_PROXYTYPE(3)\fP
+.IP CURLOPT_SOCKS_PROXYTYPE
+Socks proxy type. See \fICURLOPT_SOCKS_PROXYTYPE(3)\fP
 .IP CURLOPT_NOPROXY
 Filter out hosts from proxy use. \fICURLOPT_NOPROXY(3)\fP
 .IP CURLOPT_HTTPPROXYTUNNEL
@@ -228,10 +232,16 @@ Proxy password. See \fICURLOPT_PROXYPASSWORD(3)\fP
 HTTP server authentication methods. See \fICURLOPT_HTTPAUTH(3)\fP
 .IP CURLOPT_TLSAUTH_USERNAME
 TLS authentication user name. See \fICURLOPT_TLSAUTH_USERNAME(3)\fP
+.IP CURLOPT_PROXY_TLSAUTH_USERNAME
+Proxy TLS authentication user name. See \fICURLOPT_PROXY_TLSAUTH_USERNAME(3)\fP
 .IP CURLOPT_TLSAUTH_PASSWORD
 TLS authentication password. See \fICURLOPT_TLSAUTH_PASSWORD(3)\fP
+.IP CURLOPT_PROXY_TLSAUTH_PASSWORD
+Proxy TLS authentication password. See \fICURLOPT_PROXY_TLSAUTH_PASSWORD(3)\fP
 .IP CURLOPT_TLSAUTH_TYPE
 TLS authentication methods. See \fICURLOPT_TLSAUTH_TYPE(3)\fP
+.IP CURLOPT_PROXY_TLSAUTH_TYPE
+Proxy TLS authentication methods. See \fICURLOPT_PROXY_TLSAUTH_TYPE(3)\fP
 .IP CURLOPT_PROXYAUTH
 HTTP proxy authentication methods. See \fICURLOPT_PROXYAUTH(3)\fP
 .IP CURLOPT_SASL_IR
@@ -447,14 +457,24 @@ Timeout for waiting for the server's connect back to be accepted. See \fICURLOPT
 .SH SSL and SECURITY OPTIONS
 .IP CURLOPT_SSLCERT
 Client cert. See \fICURLOPT_SSLCERT(3)\fP
+.IP CURLOPT_PROXY_SSLCERT
+Proxy client cert. See \fICURLOPT_PROXY_SSLCERT(3)\fP
 .IP CURLOPT_SSLCERTTYPE
 Client cert type.  See \fICURLOPT_SSLCERTTYPE(3)\fP
+.IP CURLOPT_PROXY_SSLCERTTYPE
+Proxy client cert type.  See \fICURLOPT_PROXY_SSLCERTTYPE(3)\fP
 .IP CURLOPT_SSLKEY
 Client key. See \fICURLOPT_SSLKEY(3)\fP
+.IP CURLOPT_PROXY_SSLKEY
+Proxy client key. See \fICURLOPT_PROXY_SSLKEY(3)\fP
 .IP CURLOPT_SSLKEYTYPE
 Client key type. See \fICURLOPT_SSLKEYTYPE(3)\fP
+.IP CURLOPT_PROXY_SSLKEYTYPE
+Proxy client key type. See \fICURLOPT_PROXY_SSLKEYTYPE(3)\fP
 .IP CURLOPT_KEYPASSWD
 Client key password. See \fICURLOPT_KEYPASSWD(3)\fP
+.IP CURLOPT_PROXY_KEYPASSWD
+Proxy client key password. See \fICURLOPT_PROXY_KEYPASSWD(3)\fP
 .IP CURLOPT_SSL_ENABLE_ALPN
 Enable use of ALPN. See \fICURLOPT_SSL_ENABLE_ALPN(3)\fP
 .IP CURLOPT_SSL_ENABLE_NPN
@@ -467,20 +487,32 @@ Default SSL engine. See \fICURLOPT_SSLENGINE_DEFAULT(3)\fP
 Enable TLS False Start. See \fICURLOPT_SSL_FALSESTART(3)\fP
 .IP CURLOPT_SSLVERSION
 SSL version to use. See \fICURLOPT_SSLVERSION(3)\fP
+.IP CURLOPT_PROXY_SSLVERSION
+Proxy SSL version to use. See \fICURLOPT_PROXY_SSLVERSION(3)\fP
 .IP CURLOPT_SSL_VERIFYHOST
 Verify the host name in the SSL certificate. See \fICURLOPT_SSL_VERIFYHOST(3)\fP
+.IP CURLOPT_PROXY_SSL_VERIFYHOST
+Verify the host name in the proxy SSL certificate. See \fICURLOPT_PROXY_SSL_VERIFYHOST(3)\fP
 .IP CURLOPT_SSL_VERIFYPEER
 Verify the SSL certificate. See \fICURLOPT_SSL_VERIFYPEER(3)\fP
+.IP CURLOPT_PROXY_SSL_VERIFYPEER
+Verify the proxy SSL certificate. See \fICURLOPT_PROXY_SSL_VERIFYPEER(3)\fP
 .IP CURLOPT_SSL_VERIFYSTATUS
 Verify the SSL certificate's status. See \fICURLOPT_SSL_VERIFYSTATUS(3)\fP
 .IP CURLOPT_CAINFO
 CA cert bundle. See \fICURLOPT_CAINFO(3)\fP
+.IP CURLOPT_PROXY_CAINFO
+Proxy CA cert bundle. See \fICURLOPT_PROXY_CAINFO(3)\fP
 .IP CURLOPT_ISSUERCERT
 Issuer certificate. See \fICURLOPT_ISSUERCERT(3)\fP
 .IP CURLOPT_CAPATH
 Path to CA cert bundle. See \fICURLOPT_CAPATH(3)\fP
+.IP CURLOPT_PROXY_CAPATH
+Path to proxy CA cert bundle. See \fICURLOPT_PROXY_CAPATH(3)\fP
 .IP CURLOPT_CRLFILE
 Certificate Revocation List. See \fICURLOPT_CRLFILE(3)\fP
+.IP CURLOPT_PROXY_CRLFILE
+Proxy Certificate Revocation List. See \fICURLOPT_PROXY_CRLFILE(3)\fP
 .IP CURLOPT_CERTINFO
 Extract certificate info. See \fICURLOPT_CERTINFO(3)\fP
 .IP CURLOPT_PINNEDPUBLICKEY
@@ -491,10 +523,14 @@ Provide source for entropy random data. See \fICURLOPT_RANDOM_FILE(3)\fP
 Identify EGD socket for entropy. See \fICURLOPT_EGDSOCKET(3)\fP
 .IP CURLOPT_SSL_CIPHER_LIST
 Ciphers to use. See \fICURLOPT_SSL_CIPHER_LIST(3)\fP
+.IP CURLOPT_PROXY_SSL_CIPHER_LIST
+Proxy ciphers to use. See \fICURLOPT_PROXY_SSL_CIPHER_LIST(3)\fP
 .IP CURLOPT_SSL_SESSIONID_CACHE
 Disable SSL session-id cache. See \fICURLOPT_SSL_SESSIONID_CACHE(3)\fP
 .IP CURLOPT_SSL_OPTIONS
 Control SSL behavior. See \fICURLOPT_SSL_OPTIONS(3)\fP
+.IP CURLOPT_PROXY_SSL_OPTIONS
+Control proxy SSL behavior. See \fICURLOPT_PROXY_SSL_OPTIONS(3)\fP
 .IP CURLOPT_KRBLEVEL
 Kerberos security level. See \fICURLOPT_KRBLEVEL(3)\fP
 .IP CURLOPT_GSSAPI_DELEGATION