This diff adds an option to enable upnp through miniupnpc
`-DENABLE_UPNP`. By default UPnP is enabled if the miniupnpc library is
found disabled otherwise, but it might be forced `OFF` or `ON` on the
command line.
It also adds an option `USE_UPNP_DEFAULT` which can be turned ON to set
UPnP as the default to map ports (OFF by default).