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.

include/curl/curl.h

@@ -640,6 +640,7 @@ typedef enum {
                            CONNECT HTTP/1.1 */
   CURLPROXY_HTTP_1_0 = 1,   /* added in 7.19.4, force to use CONNECT
                                HTTP/1.0  */
+  CURLPROXY_HTTPS = 2, /* added in TBD */
   CURLPROXY_SOCKS4 = 4, /* support added in 7.15.2, enum existed already
                            in 7.10 */
   CURLPROXY_SOCKS5 = 5, /* added in 7.10 */
@@ -1206,7 +1207,8 @@ typedef enum {
   CINIT(SHARE, OBJECTPOINT, 100),
 
   /* indicates type of proxy. accepted values are CURLPROXY_HTTP (default),
-     CURLPROXY_SOCKS4, CURLPROXY_SOCKS4A and CURLPROXY_SOCKS5. */
+     CURLPROXY_HTTPS, CURLPROXY_SOCKS4, CURLPROXY_SOCKS4A and
+     CURLPROXY_SOCKS5. */
   CINIT(PROXYTYPE, LONG, 101),
 
   /* Set the Accept-Encoding string. Use this to tell a server you would like
@@ -1704,6 +1706,70 @@ typedef enum {
    * HTTP status code >= 300 */
   CINIT(KEEP_SENDING_ON_ERROR, LONG, 245),
 
+  /* The CApath or CAfile used to validate the proxy certificate
+     this option is used only if PROXY_SSL_VERIFYPEER is true */
+  CINIT(PROXY_CAINFO, STRINGPOINT, 246),
+
+  /* The CApath directory used to validate the proxy certificate
+     this option is used only if PROXY_SSL_VERIFYPEER is true */
+  CINIT(PROXY_CAPATH, STRINGPOINT, 247),
+
+  /* Set if we should verify the proxy in ssl handshake,
+     set 1 to verify. */
+  CINIT(PROXY_SSL_VERIFYPEER, LONG, 248),
+
+  /* Set if we should verify the Common name from the proxy certificate in ssl
+   * handshake, set 1 to check existence, 2 to ensure that it matches
+   * the provided hostname. */
+  CINIT(PROXY_SSL_VERIFYHOST, LONG, 249),
+
+  /* What version to specifically try to use for proxy.
+     See CURL_SSLVERSION defines below. */
+  CINIT(PROXY_SSLVERSION, LONG, 250),
+
+  /* Set a username for authenticated TLS for proxy */
+  CINIT(PROXY_TLSAUTH_USERNAME, STRINGPOINT, 251),
+
+  /* Set a password for authenticated TLS for proxy */
+  CINIT(PROXY_TLSAUTH_PASSWORD, STRINGPOINT, 252),
+
+  /* Set authentication type for authenticated TLS for proxy */
+  CINIT(PROXY_TLSAUTH_TYPE, STRINGPOINT, 253),
+
+  /* name of the file keeping your private SSL-certificate for proxy */
+  CINIT(PROXY_SSLCERT, STRINGPOINT, 254),
+
+  /* type of the file keeping your SSL-certificate ("DER", "PEM", "ENG") for
+     proxy */
+  CINIT(PROXY_SSLCERTTYPE, STRINGPOINT, 255),
+
+  /* name of the file keeping your private SSL-key for proxy */
+  CINIT(PROXY_SSLKEY, STRINGPOINT, 256),
+
+  /* type of the file keeping your private SSL-key ("DER", "PEM", "ENG") for
+     proxy */
+  CINIT(PROXY_SSLKEYTYPE, STRINGPOINT, 257),
+
+  /* password for the SSL private key for proxy */
+  CINIT(PROXY_KEYPASSWD, STRINGPOINT, 258),
+
+  /* Specify which SSL ciphers to use for proxy */
+  CINIT(PROXY_SSL_CIPHER_LIST, STRINGPOINT, 259),
+
+  /* CRL file for proxy */
+  CINIT(PROXY_CRLFILE, STRINGPOINT, 260),
+
+  /* Enable/disable specific SSL features with a bitmask for proxy, see
+     CURLSSLOPT_* */
+  CINIT(PROXY_SSL_OPTIONS, LONG, 261),
+
+  /* Name of socks proxy to use. */
+  CINIT(SOCKS_PROXY, STRINGPOINT, 262),
+
+  /* indicates type of proxy. accepted values are CURLPROXY_SOCKS4,
+     CURLPROXY_SOCKS4A and CURLPROXY_SOCKS5. */
+  CINIT(SOCKS_PROXYTYPE, LONG, 263),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;
 
@@ -2210,9 +2276,10 @@ typedef enum {
   CURLINFO_ACTIVESOCKET     = CURLINFO_SOCKET + 44,
   CURLINFO_TLS_SSL_PTR      = CURLINFO_SLIST  + 45,
   CURLINFO_HTTP_VERSION     = CURLINFO_LONG   + 46,
+  CURLINFO_PROXY_SSL_VERIFYRESULT = CURLINFO_LONG + 47,
   /* Fill in new entries below here! */
 
-  CURLINFO_LASTONE          = 46
+  CURLINFO_LASTONE          = 47
 } CURLINFO;
 
 /* CURLINFO_RESPONSE_CODE is the new name for the option previously known as