Move ip requesting code into a separate file and make hashing configurable at build time.

New code tries to guess a per-visitor IP address and uses
that (or its hash) to tell users apart.
This patch also adds a TAWASHI_WITH_IP_LOGGING cmake option
to enable or disable logging the IP address of your visitors
in various places (just one right now but don't assume things
to remain this way).
Also added a couple new CGI environment variables.

src/tawashi_implem/ip_utils.hpp

@@ -0,0 +1,29 @@
+/* Copyright 2017, Michele Santullo
+ * This file is part of "tawashi".
+ *
+ * "tawashi" is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * "tawashi" is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with "tawashi".  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "duckhandy/compatibility.h"
+#include <string>
+
+namespace tawashi {
+	namespace cgi {
+		class Env;
+	} //namespace cgi
+
+	std::string guess_real_remote_ip (const cgi::Env& parCgiEnv) a_pure;
+} //namespace tawashi