supply the SVPNCOOKIE via environment

if the SVPNCOOKIE is set in the enviroment authentication is skipped and
the cookie is taken from the environment instead. (addresses adrienverge#46)

doc/openfortivpn.1.in

@@ -189,6 +189,14 @@ VPN_ROUTE_MASK_... the network mask for this route
 .br
 VPN_ROUTE_GATEWAY_... the gateway for the current route entry
 
+If an environment variable
+.B SVPNCOOKIE
+is present 
+.B openfortivpn
+will skip the authentication step and use that value as cookie. It assumes
+that some other mechanism (a surrounding GUI or scripting framework) has 
+already performed the login and recieved this cookie from the remote side.
+
 .SH CONFIG FILE
 Options can be taken from a configuration file. Options passed in the command
 line will override those from the config file, though. The default config file

src/tunnel.c

@@ -771,6 +771,7 @@ int run_tunnel(struct vpn_config *config)
 		.on_ppp_if_up = on_ppp_if_up,
 		.on_ppp_if_down = on_ppp_if_down
 	};
+	char *cookie;
 
 	// Step 0: get gateway host IP
 	ret = get_gateway_host_ip(&tunnel);
@@ -785,14 +786,19 @@ int run_tunnel(struct vpn_config *config)
 
 	// Step 2: connect to the HTTP interface and authenticate to get a
 	// cookie
-	ret = auth_log_in(&tunnel);
-	if (ret != 1) {
-		log_error("Could not authenticate to gateway (%s).\n",
-		          err_http_str(ret));
-		ret = 1;
-		goto err_tunnel;
+	cookie = getenv("SVPNCOOKIE");
+	if (cookie != NULL){
+		strncpy(config->cookie, cookie, strlen(cookie)+1);
+	} else {
+		ret = auth_log_in(&tunnel);
+		if (ret != 1) {
+			log_error("Could not authenticate to gateway (%s).\n",
+		        	  err_http_str(ret));
+			ret = 1;
+			goto err_tunnel;
+		}
+		log_info("Authenticated.\n");
 	}
-	log_info("Authenticated.\n");
 	log_debug("Cookie: %s\n", config->cookie);
 
 	ret = auth_request_vpn_allocation(&tunnel);