Add UPnP support

[Ansa89]

Rebase of #969.

[alexbakker]

I would like to have UPnP configurable in Tox_Options. Something like bool upnp_enabled; would suffice.

[GrayHatter]

@Ansa89 http://cdn.meme.am/instances/23956669.jpg

[tycho]

Tried this change out locally. Sadly failed to compile against miniupnpc v1.9.20151026:

../toxcore/network.c: In function ‘upnp_map_port’:
../toxcore/network.c:130:52: warning: passing argument 6 of ‘upnpDiscover’ makes integer from pointer without a cast [-Wint-conversion]
     devlist = upnpDiscover(1000, NULL, NULL, 0, 0, &error);
                                                    ^
In file included from ../toxcore/network.c:47:0:
/usr/include/miniupnpc/miniupnpc.h:62:1: note: expected ‘unsigned char’ but argument is of type ‘int *’
 upnpDiscover(int delay, const char * multicastif,
 ^
../toxcore/network.c:130:15: error: too few arguments to function ‘upnpDiscover’
     devlist = upnpDiscover(1000, NULL, NULL, 0, 0, &error);
               ^
In file included from ../toxcore/network.c:47:0:
/usr/include/miniupnpc/miniupnpc.h:62:1: note: declared here
 upnpDiscover(int delay, const char * multicastif,
 ^

The function prototype for upnpDiscover in miniupnpc is:

MINIUPNP_LIBSPEC struct UPNPDev *
upnpDiscover(int delay, const char * multicastif,
             const char * minissdpdsock, int localport,
             int ipv6, unsigned char ttl,
             int * error);

This is probably the change that broke it: miniupnp/miniupnp@1da63fa

[tycho]

One way to fix would be to check if MINIUPNPC_API_VERSION is >= 14, and add a default TTL argument of 2 if so. Or just make it require MINIUPNPC_API_VERSION >= 14 at configure time.

[Ansa89]

@Impyy: done.
@tycho: done.
@GrayHatter: lol.

[Ansa89]

Superseded by #1539.