PROXY: implemented native proxy support

This patch creates a hook into the net_connect*() methods which call a
method to connect to a proxy.

Previous solution to send certain strings in the normal IRC dialog was
some kind of hack as most proxies require some kind of negotation.

E.g. HTTP proxies sent a 'HTTP/1.0 200 Connection established' HTTP header
and clients have to wait for it.  Else, sent bytes of the following IRC
login will be dropped silently.

With old method, it is also impossible to tunnel SSL IRC connections
through the proxy as proxy speaks plain text or a special protocol while
e.g. 'CONNECT ... HTTP/1.0' will be encrypted with key of IRC server.

There are further enhancements possible: the whole net_connect stuff
should be made asynchronously. Currently, only the hostname is resolved
in the background (which makes little sense of local proxies usually).

src/core/network-proxy.c

@@ -0,0 +1,30 @@
+/*	--*- c -*--
+ * Copyright (C) 2008 Enrico Scholz <enrico.scholz@informatik.tu-chemnitz.de>
+ *
+ * This program 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; version 2 and/or 3 of the License.
+ *
+ * This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "module.h"
+
+#include "network-proxy.h"
+#include <string.h>
+
+struct network_proxy *
+network_proxy_create(char const *type)
+{
+	if (type==NULL)
+		return NULL;
+
+	g_error("unsupported proxy type '%s'", type);
+	return NULL;
+}