MINOR: Add sample fetch to detect Supported Elliptic Curves Extension

Clients that support ECC cipher suites SHOULD send the specified extension
within the SSL ClientHello message according to RFC4492, section 5.1. We
can use this extension to chain-proxy requests so that, on the same IP
address, a ECC compatible clients gets an EC certificate and a non-ECC
compatible client gets a regular RSA certificate. The main advantage of this
approach compared to the one presented by Dave Zhu on the mailing list
is that we can make it work with OpenSSL versions before 1.0.2.

Example:
frontend ssl-relay
        mode tcp
        bind 0.0.0.0:443
        use_backend ssl-ecc if { req.ssl_ec_ext 1 }
        default_backend ssl-rsa

backend ssl-ecc
        mode tcp
        server ecc unix@/var/run/haproxy_ssl_ecc.sock send-proxy-v2 check

backend ssl-rsa
        mode tcp
        server rsa unix@/var/run/haproxy_ssl_rsa.sock send-proxy-v2 check

listen  all-ssl
        bind unix@/var/run/haproxy_ssl_ecc.sock accept-proxy ssl crt /usr/local/haproxy/ecc.foo.com.pem user nobody
        bind unix@/var/run/haproxy_ssl_rsa.sock accept-proxy ssl crt /usr/local/haproxy/www.foo.com.pem user nobody

Signed-off-by: Nenad Merdanovic <nmerdan@anine.io>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 90834a7..03388e3 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -12631,6 +12631,15 @@
   ACL derivatives :
     req_rdp_cookie_cnt([<name>]) : integer match
 
+req.ssl_ec_ext : boolean
+  Returns a boolean identifying if client sent the Supported Elliptic Curves
+  Extension as defined in RFC4492, section 5.1. within the SSL ClientHello
+  message. This can be used to present ECC compatible clients with EC certificate
+  and to use RSA for all others, on the same IP address. Note that this only
+  applies to raw contents found in the request buffer and not to contents
+  deciphered via an SSL data layer, so this will not work with "bind" lines
+  having the "ssl" option.
+
 req.ssl_hello_type : integer
 req_ssl_hello_type : integer (deprecated)
   Returns an integer value containing the type of the SSL hello message found