[MEDIUM] add support for RDP cookie persistence

The new statement "persist rdp-cookie" enables RDP cookie
persistence. The RDP cookie is then extracted from the RDP
protocol, and compared against available servers. If a server
matches the RDP cookie, then it gets the connection.
diff --git a/src/proxy.c b/src/proxy.c
index f6c142f..4e75b59 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -677,6 +677,12 @@
 		s->req->analysers |= AN_REQ_WAIT_HTTP | AN_REQ_HTTP_PROCESS_BE | AN_REQ_HTTP_INNER;
 	}
 
+	/* If the backend does requires RDP cookie persistence, we have to
+	 * enable the corresponding analyser.
+	 */
+	if (s->be->options2 & PR_O2_RDPC_PRST)
+		s->req->analysers |= AN_REQ_PRST_RDP_COOKIE;
+
 	return 1;
 }