Add UPnP support.

Feedback is in logs as suggested by skid_au. The checkbox is still there, but
mostly for people who would like to opt out (unfortunately, I can not be sure
how this feature may behave for some routers - there's a hell of a lot of bad
UPnP implementations.)

The Visual Studio stuff is a little messy, so I apologize if anything is a bit
off. I tested most configurations and it worked.

I also tested CMake on Debian Wheezy, Ubuntu Saucy, and Mac OS X Mountain Lion.
All seemed to be OK.

Externals/miniupnpc/src/miniupnpc.h

@@ -0,0 +1,130 @@
+/* $Id: miniupnpc.h,v 1.32 2013/02/06 14:44:42 nanard Exp $ */
+/* Project: miniupnp
+ * http://miniupnp.free.fr/
+ * Author: Thomas Bernard
+ * Copyright (c) 2005-2012 Thomas Bernard
+ * This software is subjects to the conditions detailed
+ * in the LICENCE file provided within this distribution */
+#ifndef MINIUPNPC_H_INCLUDED
+#define MINIUPNPC_H_INCLUDED
+
+#include "declspec.h"
+#include "igd_desc_parse.h"
+
+/* error codes : */
+#define UPNPDISCOVER_SUCCESS (0)
+#define UPNPDISCOVER_UNKNOWN_ERROR (-1)
+#define UPNPDISCOVER_SOCKET_ERROR (-101)
+#define UPNPDISCOVER_MEMORY_ERROR (-102)
+
+/* versions : */
+#define MINIUPNPC_VERSION	"1.8.20130503"
+#define MINIUPNPC_API_VERSION	9
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Structures definitions : */
+struct UPNParg { const char * elt; const char * val; };
+
+char *
+simpleUPnPcommand(int, const char *, const char *,
+                  const char *, struct UPNParg *,
+                  int *);
+
+struct UPNPDev {
+	struct UPNPDev * pNext;
+	char * descURL;
+	char * st;
+	unsigned int scope_id;
+	char buffer[2];
+};
+
+/* upnpDiscover()
+ * discover UPnP devices on the network.
+ * The discovered devices are returned as a chained list.
+ * It is up to the caller to free the list with freeUPNPDevlist().
+ * delay (in millisecond) is the maximum time for waiting any device
+ * response.
+ * If available, device list will be obtained from MiniSSDPd.
+ * Default path for minissdpd socket will be used if minissdpdsock argument
+ * is NULL.
+ * If multicastif is not NULL, it will be used instead of the default
+ * multicast interface for sending SSDP discover packets.
+ * If sameport is not null, SSDP packets will be sent from the source port
+ * 1900 (same as destination port) otherwise system assign a source port. */
+LIBSPEC struct UPNPDev *
+upnpDiscover(int delay, const char * multicastif,
+             const char * minissdpdsock, int sameport,
+             int ipv6,
+             int * error);
+/* freeUPNPDevlist()
+ * free list returned by upnpDiscover() */
+LIBSPEC void freeUPNPDevlist(struct UPNPDev * devlist);
+
+/* parserootdesc() :
+ * parse root XML description of a UPnP device and fill the IGDdatas
+ * structure. */
+LIBSPEC void parserootdesc(const char *, int, struct IGDdatas *);
+
+/* structure used to get fast access to urls
+ * controlURL: controlURL of the WANIPConnection
+ * ipcondescURL: url of the description of the WANIPConnection
+ * controlURL_CIF: controlURL of the WANCommonInterfaceConfig
+ * controlURL_6FC: controlURL of the WANIPv6FirewallControl
+ */
+struct UPNPUrls {
+	char * controlURL;
+	char * ipcondescURL;
+	char * controlURL_CIF;
+	char * controlURL_6FC;
+	char * rootdescURL;
+};
+
+/* UPNP_GetValidIGD() :
+ * return values :
+ *     0 = NO IGD found
+ *     1 = A valid connected IGD has been found
+ *     2 = A valid IGD has been found but it reported as
+ *         not connected
+ *     3 = an UPnP device has been found but was not recognized as an IGD
+ *
+ * In any non zero return case, the urls and data structures
+ * passed as parameters are set. Donc forget to call FreeUPNPUrls(urls) to
+ * free allocated memory.
+ */
+LIBSPEC int
+UPNP_GetValidIGD(struct UPNPDev * devlist,
+                 struct UPNPUrls * urls,
+				 struct IGDdatas * data,
+				 char * lanaddr, int lanaddrlen);
+
+/* UPNP_GetIGDFromUrl()
+ * Used when skipping the discovery process.
+ * return value :
+ *   0 - Not ok
+ *   1 - OK */
+LIBSPEC int
+UPNP_GetIGDFromUrl(const char * rootdescurl,
+                   struct UPNPUrls * urls,
+                   struct IGDdatas * data,
+                   char * lanaddr, int lanaddrlen);
+
+LIBSPEC void
+GetUPNPUrls(struct UPNPUrls *, struct IGDdatas *,
+            const char *, unsigned int);
+
+LIBSPEC void
+FreeUPNPUrls(struct UPNPUrls *);
+
+/* return 0 or 1 */
+LIBSPEC int UPNPIGD_IsConnected(struct UPNPUrls *, struct IGDdatas *);
+
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
+